Note: Descriptions are shown in the official language in which they were submitted.
6 ~ 4
BACKGROUND OF THE INVENTION
The present invention relates. to proces.ses for the production
of key intermediates in the syntheti.c pathway leadi.ng to naturally occurring
cytokini.ns, particularly ~an~-zeati:n and dihydrozeatin.
Cytokinins are naturally occurring 6-s.ubs.ti.tuted aminopurines,
which are plant hormones known for th.eir biologi:cal acti:vity on plant
growth. The more important of these effects are the ab.ilities to induce
ce11 division and regulate di.fferentiation i.n exci.s.ed plant tissue. Zeatin
is one of the most acti.ve forms of the cytok.i:nins.
: 10 Owing to the diffi:culty in i.solati:ng even mi;nute. quantiti.es of
the naturally occurring cytoki.nins, researchers have turned their efforts
to investigating synthetic path~ays to form bi:ologically acti.ve 6-
substituted ami.nopuri.nes. For ~an~.-zeatin, much. of the research has
been focus~ed on a suitable synthesi.s of ~a~-3-h~droxymethylbut-2-
: 15 enylami.ne (VI.), th.e h.ighly functi;onali:zed unsaturated side chain ~hi.ch.can be condensed, by kno~n methods, wi.th 6-ch.lorQpuri`ne to fcrm ~ai~-
: ~ zeati.n. In the case of dihydrozeati:n. (II~, th~ des-i:red si:de. chai:n i5
3-hydraxymethyl butyl ami.ne ( VI I ) .
; ~ Thu~ far, kna~n methods. for the synthesi`s of ~an~-3-
hydroxymethylbut-2-enylamine fall into one of the following th.ree
: categori.es:
1. A multi.step synthe~.is involvi.ng an allyli`c 6romi.natïon
and starti.ng with tiglic acid, see. for example G. Shaw
et alr,J. Ch.em. SQG. (C~, q21~ 1966, and D. S. Letham et
al~,Phytochemi:stry, 1Q, 2n77, lq71;
2. A multi.~tep synthes:is. starting from acetone and cyanoaceti.c
aci:d and i.nvolvi.ng allylic 6rominati`on, see. for example
D. S. Letham et al., Aust. J. Chem., 22, 205, 1969; and
3. A Ga~riel ph.thali.mi:de synthesi:s far the selecti:ve allyli`c
3n ami.nation, s.tarti:ng from i:soprene or an isoprenoid hali.de,
see for example M. Ohsugi: et al.,Agr. Biol. Chem., 38,
1925, 1974, R. Mornet et al.,Tetrahedron Lett., 167, 1~77,
J. Corse et al.,Synthesi:s, 618, 1972, and G. Desvages et
-- 2 --
I 1 ~26~4
al., Bull. Soc. Chim. Fr., 3329, 1~6~.
Generally the methods of the fi:rst tWQ categori.es involve
many s.teps:, provi`de low yields and requi:re the diffi:cult separati'on of
geometri:c i.samers of the ~ unsaturated nitri:le necessarily produced
by an allyli:c ~rami:nati:on. The thi'rd method involves an unstable dibromide
intermediate and requires an undesirable recrystallizati:on step in order
to separate an i.ntermedi`ate mi:xture.
By th.e method descri:'bed by D.S. Letham et al., Aust. J.
; ChRm., 22, 205, 196~, it i`s possi:b.le to selecti:vely reduce ~an~-3-
h~droxymethylbut-2-enenitrile (VI~ ) to ~han~-3-hydroxymeth~lbut-2-
enylami.ne (VI) using 2-tetrahydropyranyl as: a protect;ng group for the
hydroxyl functi:on. It mus.t be poi.nted out that, on the basis of the
pres.ent i.nventars expe.ri:ences, li.thium alumi:num hydride reduction of
; the 2-tetrahydropyranyl ether of hydroxyni:trile (VIII), after the pro-
cedure of Leth.am et al., invari:ably leads- to a complex mixture of
products, ;.n ~hi.ch saturated ami:nes ~re ~found to be main const;tuents.
Othe~wi.se the hydroxynitri.le (VII:I~ can be exhaustively reduced to form
3-hydroxymethylbutylami.ne (VII~. Condensat;on of the unsaturated
or saturated ami.ne w;th 6-chloropuri.ne, by for example the method of
Leth.am et al., i.s kno~n to yield the ~n~-zeati.n or dihydrozeatin.
I:t is therefore an object of the present invention to provi.de
processes for the preparati:on of the unsaturated amino alcohol (VI) and
the saturated ami.no alcoh.ol (VII:~, whi:ch can th.en be transformed by known
methods to ~n~.-zeatin and dihydrozeatin respecti'vely.
Tn a process descri:bed i:n Uni:ted States Patent No. 3,960,923
is.sued to De5imone, ~ unsaturated nitri:les can be prepared by reaction
of a ketone and acetonitri:le i:n the presence of a base. However, as
reported i.n a publi.cati:on b.y S. A. Di:Bi:ase et al., (Synthesis, 629, 1977)
and as s:upported by th.e poor yield data presented by De5imone, this base
catalyzed condensati:an fails or gives ver~ poar yi:elds wi:th methyl ketones.
Si:nce th.e starti:ng materi:al for the synthesis of the desired
unsaturated ami.no alcohol ('VI) or saturated am;:no alcohol (~ , using the
reactian scheme of DeSi.mone, i.s necessarily a methyl ketone, this process
~ 1 ~2~
is not feasi~le for an unact;vated methyl ketone.
SUMMARY'OF'TH NVENTI.ON
The inventor has di:scovered that whereas a methyl ketone such
as acetone cannot be sati:sfactorily condensed wi:th acetoni.~ri:le, an
acetal of pyruvaldehyde, whi.ch i.s: an a~a-di:alkoxymethyl s.ubstituted
methyl ketone, can be condensed with acètoni.tr;:le with surprisi.ngly good
results. The pres:ence of the acètal group h.as a number of unexpected
effects on the condensati`on reacti`on.
Fi:rstly, the fact that tfie reaction proceeds at all suggests
that the acetal group has a stabili:z;ng effect on the polari:zing carbonyl
group of the pyruvaldefiyde acetal. Secondly, the acetal group, being a
hulky s.uhs.t;:tuent, i:nfluences: the stereocfiemistry of the condensation
react;on to gi:ve the preferred ~an~- configurati:on in good yield. Thirdly,
th.e acetal group can be hydrolyzed and sufisequently reduced to yield the
3-hydroxymethyl functi:anality i.n the target intermediates for the synthesis
of ~an~.-zeati.n CI:) and dihydrozeatin (II)~ namely t~a~-3-hydroxymethylbut-
2-enylamine ~VI.~ and 3-hydroxymethylbutylamine (:VII) respectively.
Thus, in accordance wi:th the present invention a pyruvaldehyde
acetal i.s condensed with acetonitrile in the presence of a strong base.
Preferably th.e hase i.s s;elected from tfie group consi:sti'ng of an alkali
alkoxide, sodi:um hydroxi.de and potassi'um hydroxide. The reaction proceeds
at elevated temperatures;, preferably at reflux temperature. Excess
acetoni:tr;:le is preferably i.ncluded as the solvent for the reaction.
The base catalyzed condensati:on product formed is a c~d, ~han~
mi.xture of th.e correspond;.ng acetal of 3-formylbut-2-eneni'trile (IV), a
novel product. As mentioned previ:ous1y, the reaction proceeds regio-
selecti.vely favouring the ~han~-i:somer.
Broadly stated, the present invention provides a process
compri.s.ing: reacti.ng a pyruvaldehyde acetal having the general formula
~ ~72~
R~ /R
Q \ /0
CH
Cl w o
CH3
where. R ;s an alkyl, cycloalkyl, s~ub.sti`tuted al:kyl~ or alkenyl group
havi.ng from ab.out l - lO carhon atoms~ or part of a methylene chain
in a cyclic acetal of a 5 - lO membered r~ng structure, with acetonitrile
i:n the presence of a strong 6ase at an e1evated temperature for a time
su~fi;ci.ent to form an acetal af 3-formylbut-2-enenitrile.
I:n a further aspect of tfie invention~ the acetal of 3-formylbut-
2;e.nenitri.1e ~IV) i.s acid hydrolyzed, pre~e.rably i:n an aqueous solution
; of a mine.ral acid or in an aqueous solu~ion of a m;neral acid with a water
mi.sclble organi:c solvent sucn. as me~hanol, ethanol, or tetrahydrofuran, to
1.0 form ~an~-3-formylbut-2-eneni.tri:1e ~V~, also a novel product. Dis-
:
- 4a -
~ ~ 7264~
tillation of the crude product yi.elds~ exclus;:vely the ~a~-i.somer,
eli.mi.nati.ng the need for separating geometric i.somers at any poi:nt ;n
the overall syntheti:c path~ay. Pres.umabl~ the c,~.-is.omer, i.f formed,
~urth.er reacts i:ntra- or i:ntermole.cularly to form a h~:gher boili.ng
fraction.
The aldehyde group of ~an~-3-form~lbut-2-enenitri:le can be
selectively reduced i:n the presence of sodi.um ~orohy-dri.de in the presence
of a suita~le s:olvent s:uch as: met~lanol, eth.anol, or tetrahydrofuran to
yi:eld ~on~-.-3-hydroxymethylbut-2-enenitrile (VII:I~ wi.thout csntamination
of th.e saturated alcohol. By mask.i:ng the allyl;:c hydroxy function ~ith a
bulk~ s.ubsti:tuent~ th.e ~ unsaturated n:i:tri.le (VIII~ can be further
selectively reduced with lithi.um alumi:num hydride to form ~on~-3-
hydroxymethylbut-2-enylam;ne (VI). A number of bulky sily groups, such
as ~Qh~-butyldi.methylsilyl group, see for example E.J. Corey et al., J. Am.
Chem. Soc., 94, 6190, 1972, have been found to be effective for this pur-
pose. The uns.aturated ami.no alcohol ~VI:) thus formed can be subsequently
: condens.ed wi.th 6-chloropurine (.IX~ to form ~La~-zeati.n by known methods,
s.ee for example G. ~haw et al.
Other~i.se, the ~on~-3-formyl~ut-2-enenitrile (V~ can be
2a exh.austively reduced wi:th a sui.table reducing agent to form 3-
h~droxymethylbutylami:ne (VI:I.). The saturated amino alcoh.ol (VII~ can be
condensed wi.th 6~chloropuri:ne, for example, by the method of Letham et al.
to form dihydrozeati.n (II). Preferably the reduci:ng agent is a metal
hydri.de-transi~tion metal s.alt system such as NaBH4-CoC12 6H20, although
a metal hydri.de such as Li.AlH4, or a s.ui:table hydrogenation catalyst in
the presence of hydrogen can also be employed.
DESCRI.PTION OF THE DRAWI:NGS
Fi.gure 1 i.s a formula s.heet showi:ng structural formulas and
names for the compounds referred to in the speci:fication, and
Fi:gure 2 i.s a reacti.on sheet s.h.owi:ng an example of the reac~ions
descri~ed i:n the s.pec;:fi:cati:on.
~ 77~6~
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention involves a base catalyzed condensation
of a pyruvaldehyde acetal with acetonitrile to form the corresponding
acetal of 3-formylbut-2-enen;trile, which is subsequently hydrolyzed in
acidic conditions to form ~a~-3-formylbut-2-enenitrile. Figure 2 shows
the reactions involved in the present invention starting from pyruvaldehyde
dimethyl acetal.
A large number of pyruvaldehyde acetals are s.uitable as a
starting material for the process. Generally the structural formula for
the pyruvaldehyde acetal i5
R R
\C/
C = Q
CH3
~here R i.s. an alk~l, cycloalkyl, su~stituted alkyl or alken~l group,
having from a~out 1 - 10 carhon atoms, or R i5 a methylene group in a
polymethylene chai.n -(:CH2~n- i:n a cycli`c acetal having a 5 - 1~ memb.ered
ri.ng structure. Thus the general formula as given i:s meant to represent
both. substituted acetals, such as~ pyruvaldehyde dimethyl acetal, and
cyclic acetals such as 2-acet~yl-1,3-di.oxan.
A number of pyruva'ldehyde acetals- are commerci:ally avai.lable,
however they can also be prepared by reacti.'ng 1,3-dihydroxy-2-propanone
wi.th monohydri:c or di:hydri.c alkanols 6y, ~or example, the methqd of
S. K.. Gupta, J. Qrg. Chem., 41, 2642, 1~76'.
The condensation reacti:on proceeds wi:th stoi:chi:ometri.c
quanti.ties. of acetoni.tri:le and the pyruvaldehyde acetall however i:t i.s
usually desi.rab.le to use a large excess of acetoni:tri'le to act as- both
a reagent and a solvent. The excess- acetonitrile can be recovered at
the end of the reacti.on and recycted. I'ncreas:ed ~yi'elds are generally noted
\
' 1 726~4
when acetonitrile is. used in large exces.s..
The base us:ed ta catalyze the reacti.on i;s. ~ell knawn i.n con-
densation reactions. Preferably the base is an alkali: metal alkoxide
such as sodium or potassium methoxi.de or s.odium t-butoxi:de. Mos.t
preferably the alkali. alkoxi:de carri:es the same alk~l group as the R group
in the pyruvaldehyde acetal. For i`nstance, sodi.um or pokassi:um methaxi.de
are preferred bases for the condensation of acetonitri:le ~ith pyruvaldehyde
dimethyl acetal.
Sodium or potassi.um hydroxi.de ma~ be us:ed i:n lieu of the
lO alkoxide, however the yi.eld of th.e hydro~ide catalyzed reacti.on i:s aften
inferior.
The amount of base us.ed i.s. prefe.rab.ly i:n th.e range of 0.2 to
1.2 moles. per mole of pyruvaldehyde acetal. Th.e amount of bas-e. used has
no signi.ficant effect on the final ~i:eld of the a,~-uns:aturated nitrile,
15 however the rate of addi.ti.on of th.e cyanometh.ylene anion to the p~ruvaldeh~de
acetal and th.e suhsequent dehydrati:on to gi:ve. the ~ unsaturated nitrile
i.ncreases wi.th i.ncreasi.ng quanti:ti.es! of base. For example, ~hen pyruvaldehy-de
dimethyl acetal was allowed to react ~ith. acetoni:trile i.n th.e presence of
one equivalent of sodium meth.oxi:de at reflux tempera.ture ~or about 2 hours
20 the product mi.xture yi.elded ahout 25~ of the te.rti~ary alcohol and 4~% ofunsaturated nitri.le. The yi:eld af the alcohol ~as found to decrease
with both an i.ncreas.e i.n the amount of base, as- ~ell as ~ith an increase
in the reactian ti.me.
: The base may be fi.rst i:ntroduced i:n the acetonitri:le and the
25 mi.xture subsequentl~ brought up ta reflux temperature. A solution Qf
the pyruvaldeh~de acetal i.n acètonitrile i.s then added dropwise over about
2 - 3 hours and the reacti.on mi.xture refluxed for ahout 3 - 5 hours
longer. Alternati:vely, the pyruvaldehyde acetal, base and acetanitri.le
may be charged i.n one reacti:on vessel and whole refluxed until the
completi.on of the reacti.on.
! 1 7~6~1
The reaction time has been found to be largely variable
with the R-substituent of the pyruvaldehyde acetal. The addition oF
the cyanomethylene anion, which is generated in situ, to the
pyruvaldehyde acetal may be completed within about one hour, however an
extended reaction time of as much as 10 hours may be required to ensure
the dehydration of the primary adduct to give the desi:red ~,~-unsaturated
nitrile. Generally, a pyruvaldehyde acetal which carries a bulky acetal
substituent, such as diisopropylacetal or di.oxan, requires a longer time
for the dehydration of the primary adduct.
At the completion of the reacti:an, the res:ultant m;.xture i.s
washed with water to remove the major;:ty of base and acetami:de, a
byproduct resulting from the hydrati:on of acetoni:tri.le i:n the presence of
base. Usually, the amount oF water used for ~as.hi.ng should be less than
one tenth of the volume of acetoni:tri:le us.ed for the reacti:on sa that the
organi.c phase remai.ns immiscible wi:th the aqueous pKase.. Th.e organic
phase may b.e washed agai:n ~i.th wate~. The combi;ned e.xtract is back-
~ashed wi.th lo~.-boi:li:ng organi:c solvent tQ gi`ve addi`ti.onal crop of
unsaturated ni.trile. Evaporati:on of tKe s.olvent under reduced
press.ure gi.ves a crude li.qui:d product. Th.e comb.ined crude product i.5
distilled at reduced pressure to gi:ve a c4~,~nan~ miXture of 3-formylbut-
2-enenitrile acetal (.I.V). The se~aration of th.e i:someri:c mi`xture,
which contains about 10 - 15% of C4~. form is nat necess:ary since. the
subs.equent transFormati:on of the acetal to the aldehy-de produces
~nan~-3-formylbut-2 eneni.trile C~) as the mai;n product,wh.ich can he
eas~ily di.stilled wi:thout contami:nati:on of i:ts; c~-isomer.
The above formed i.someri:c mixture of the acetal of 3-
farmylbut-2-eneni.tri:le (I:V) can be hydrolyzed under acidi`c conditions
to remove the acetal group. The aci.di:c sQlution us.ed i`s: generally an
aqueous s.olut~on of a mineral or strong organi:c acid. Alternately the
acidi:c solution can be an aqueous solut;on of a mineral or suitable strong
organi.c acid together ~ith a water mi:scible organic solvent to solubilize
-- 8 --
I 1 726 ~ ~
the start~ng material~ A 5% perchloric'aci:d solub~on has been found
to be sui:table for the hydrolys.is reaction. Other suitable acids
include hydrochl.oric~ sulphuri:c and oxalic acid
Acid hydrolysis will proceed at ambient temperatures, however
the reaction time may be reduced from about 10 hours to about one hour
by hydrolyzing at a reflux temperature.
' The product from the acid hydrolysis is extracted with low-
boiling organic solvent, such as methylene chloride, to give crude hydrolyzed
products. The organic extract ;s dried with an appropri.ate drying agent
10- and then concentrated to give crude aldehyde which can be distilled either
at atmospheric pressure (182 - 187C at Edmonton, Alberta, Canada~
or at reduced pressure and a low temperature.
The novel aldehyde of thi.s i.nvention, namely ~an~-3-
formylbut-2-enenitrile (V), has been prepared and shown to have the
following phys.ical parameters:
b.p. 71/11 mm Hg; NMR (60 Mi-lz TM~ as internal standard i.n
CDC13 solventl ~ 9.73 (singlet, lil), ~ 6.40 (quartet, J =
1.5 Hz, lH) and ~ 2.10 (doublet, J = 1.5 Hz, 3Hl, IR (liqui'd
fi;lm) 2843, 2730, 2220, 17Q5, 135a, 1195, 1027 and 830 cm 1.
20. 2,4-dini.trophenylhydrazone deri.vative, m.p. 276 (decomp.~;
bis(~hon~-3-formylb.ut-2-enenitri.le)hydrazoneg m.p. 160 -
161C. The li.qui.d aldehyde and the two crystalli'ne hydrazone
derivati.ves gave correct elemental analys.i`s and mass. spectral
data.
The aldehyde of this i.nventi:on can be selectively reduced
with sodi.um borohydri.de i.n the presence of'a low-boi'li:ng alcohol as
solvent, such as methanol or ethanol, to gi:ve ~a~-3-hydroxymethylbut-
2-eneni.tri.le (VIII~ wi'thout contaminati'on of a saturated alcohol resulting
from reducti:on of the conjugated carbon-carbon double bond. The fact
30- that the conjugated carbon-carbon bond remai'ns. i.'ntact with Na~H4 reducti:on
sugges.ts that keto-enol tautomeris.m i.n tiie c~,~-unsaturatecl aldehyde is
i.nsi:gni:ficant. Th.ïs i.s clearly due to its extended conjugatlon ~i.th the
~ ~ 72~4~
nitrile group.
The ~nan~-3-hydroxymethylbut-2-enenitrile (VIII~ formed by
this selecti:Ye reduction ;s a natural product Found as the alcohol moiety
of many plant li:pids-.
The hydroxyn;`trile (VIrr) thus ~ormed can be further trans-
formed to ~a~--3-hydroxyme~hylbut-2-enrlami`ne (VI) in very low yield by
the method of Letham et al. Alternati'vel~, it has been found that a number
of s;lyl ethers of ~an~-3-hydroxymetnylbut-2-enenitr;le (VIII) can be
smoothly transformed to the unsaturated amine (VI) ~ith a LiAlH4 reduction
in the presence of diethyl ether solvent. The bulky silyl groups are
able to mask the allylîc fiydroxyl function i'n the unsaturated nitrile
(VIII) from the metal hydride complexati'on and thus prevent hydride
reduction of the carbon-carbon double bond. Suitable silyl reagents include
t-butyld;'methyls;lyl hali'des and a range of tri-alkylated silyl halides
~15 which can protect the hydrox~l function from metal chelation and resist hydride
reduct;on. The method for their use is disclosed by E. J. Corey et al.,
J. Am. Chem. Soc., '~4, 6190, 1972. The protective silyl group of the
protected nitrile (X~ is removed by ac;d hydrolysis, preferably in a
solut;on consist;ng of 2 normal equivalents of an aqueous mineral acid and
20~ water misci~le organi'c solvent, such as methanol or tetrahydrofuran. The
hydrolysate is ~ashed ~i'th ether or a low-boiling water immiscible organic
` ; solvent. The product unsaturated am;ne ~VI) ~s recovered from the aqueous
solut;on as a salt of an oxo acid or hydrohalide, such as a sulfate or
chloride.
t is also possible in accordance ~;th the invent;on to
reduce the ~ unsaturated aldehyde (V) to (+~ -3-hydroxymethylbutylamine
(VII~ w;th a metal hydride-trans;tion metal salt system or with an
appropr;a~e metal hydride, or ~ith the uptake of four moles of hydrogen by
means of catalyt;c hydrogenat;on. The reduct;on method used in the
invention ;.s prefera~ly a m;xed sodium borohydr;de-metal salt system,
such as NaBH4 CoC12-6H2Q, ;n the presence of a low boiling alcohol.
- lû -
~ ~ ~2~4~1
The reduction can be performed by adding five to ten moles excess of
sodium borohydride to a 1:1 mole equivalent mixture of cobaltous
chloride hexahydrate and the ~ unsaturated aldehyde. Preferably
however, the aldehyde is first reduced with one mole equivalent of the
metal hydride and then one mole equivalent of CoC12-6H20 is introduced,
followed by adding progressively a large excess of sodium borohydride.
The use of this metal hydride-transition metal salt reducing agent has
been documented by T.Satoh et al., Tetrahedron Lett., 52, 4555, 1969.
The ~na~-3-hydroxymethylbut-2-enylamine (VI) or its salt (XI)
and the 3-hydroxymethylbutyl amine (VII) can be condensed with 6-
chloropurine (IX) to form ~o~-zeatin (I) and dihydrozeatin CII~
respectively. Experimental procedures and conditions have been ~ell
documented in the literature and thus wi;ll not be disclosed herein. See
for example Letham et al., Aust. J. Chem. 22, 205, 1~69.
It should be pointed out that the 3-hydroxymethylhutylamine
(VII) formed by the above described proce~s has an asymmetric centre.
Thus the dihydrazeatin formed therefrom is a racemic mixture. The
production of racemic dihydrozeatin and opti:cally pure enantiamers
has been reported by T. Fujii et al., Tetrahedron Lett., 30, 3a75, 1~72.
The following examples further illustrate the nature of the
inventian and the manner of practicing the ~ame:
Example 1. 3-~Formyl~ut-2-enenitri:le dimethylacetal
A mixture of 6~0 ml of acetonitrile and 23 g of s-odi`um
methoxide was~ heated to reflux temperature under nitrogen purge and
mechanical stirring. To the a~ove mixture was- added drap~i~se, through
a pressure equalizing addition funnel9 50 g af pyruvaldehyde di`methylacetal
in 200 ml of acetonltrile aver a peri:od of tfiree hours. At the campletion
of addition, the whole mixture ~as heated to re~lux temperature for a
further five hours and then allo~ed ta caol to room temperature. The
resultant mixture was then shaken t~ice with a ~0 ml porti`on of ~ater
1 1 _
~ ~2~44
to remove most of the base. The acetonitrile solution was separated and
concentrated at 40C under reduced pressure of about 15 mm Hg to give
crude ~ unsaturated nitrile. The combined aqueous extract was backwashed
twice with a 50 ml portion of ether to recover an additional crop of
the product. The combined crude product was distilled under reduced
pressure of 0.05 to 0.5 mm Hg at a distilling head temperature from
32 to 70C yielding 41.8 g of ~,~-unsaturated ni.tri.le (7Q% yi.eld).
This product was a colorless liqui.d which consists of about 11% of
c~-3-formylbut-2-enenitrile dimeth~'lacetal and 89% of ~hàK~-isomer, as
analyzed~hy gas chromatography.
; Example 2. 3-(1,3-Di.oxan-2-yl)but-2-eneni`trile.
A mi.xture of 4.2 g of sodium me.thoxi:de and 120. ml of
aceton1.trile was stirred mechani.call~ and heated ta reflux under nitrogen
purge. Tv. thi.s was then.added dropwi;s-e 10 ~ of 2-acetyl-1,3-dioxan in
50 ml of acetoni.tri.le aver a periPd of 2 hours and followed hy a 10 hour
reflux peri.od. After cooling, the reacti:on mixture was~ washed twice with
a 10. ml portion of water. The organic phase was then concentrated under
reduced pressure at about 4QC to give. the crude ~ unsaturated nitri.le.
The aqueous phase was washed twice ~i.th. 20 ml pQrtiOnS af ether and the
extracts. combined, dried, and evaporated to gi've an addi:tional crop
: of the product. The comb.i.ned crude prQduct ~a.s disti`lled under a re-
duced pressure of G.2 mm Hg and collected i`:n a ~oi~ ng range of 72 to
78C, gi.vi.ng 7.~4 g (65% yi.eld) of 3-(1,3-diPxan-2-yl~but-2-enenitrile
: as a colorless li:qui.d. Both NMR and GLC analyses of the above
disti.llate s.howed that the liqui'd consi.sted o~ 88~ of tha~ and 12,~
of the c~s-~isomer of 3-C1,3-di.axan-2-yl)but 2-eneni:tri.le.
I:n a separate experiment in whi:ch the reflux period ~as
s.hortened to 5 hours after addi.tion of the acetylacetal, the primary
adduct , 3-(~1,3-di:oxan-2-yl~-3-hydroxyhutyronitri:le, b.p. 92-lUQC/0.2
3Q mm Hg, was i:s.olated i:n about 25% yield.
- 12 -
g l 7264`~1
Example 3. 3-Formylbut-2-enenitrile dimethylacetal
As in example 1, acetals of 3-formylbut-2-enenitrile can also
be prepared by directly heating a mixture of an acetal of pyruvaldehyde
and sodium methoxide in a large excess of acetonitrile. Thus a mixture of
pyruvaldehyde dimethylacetal (50 g), sodium methoxide (11.5 g, 0.5
equivalent) and acetonitrile (600 ml) was heated to reflux temperature
for 12 hours. After cooling, the resultant mixture was worked up in a
manner similar to that described in example 19 giving 40 g of 3-formylbut-
2-enenitrile dimethylacetal.
The results obtained from the condensation of other pyruvaldehyde
acetals ~ith acetonitrile are summarized in Table I.
- 13 -
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- 14 -
~ 1 726~`~
Example 4. ~a~-3-Formylhut~2-enenitri~le (V)
FQrt~-three grams of 3-formylhut-2-enen;tr;le dimethylaceta'l
prepared according to Example 1 or 3 (`described above) was d;ssolved in a
methanolic hydrochloric acid solut;on, prepared by mixing 80 ml of
methanol, 500 ml water, and 26 ml of 12N nydrochloric acid. The mixture
~as heated to reflux for 1 hour. After cool;'ng, the mïx~ure was extracted
w;th three 300 ml portions of methylene cfilor;~de. The organ;c layer
~as separated and ~ashed ~itfi br;ne, dr;`ed over Na2S04 and evaporated
to remove the solvent. The residue was disti`lled at 70 - 73C at a re-
0 duced pre~sure of 11 mm Hy to give 23.5 g (81% y;eld) of ~an~-3-formylbut-
2-enenitr;le. The ~an~-aldehyde was free from the c~-isomer as evidenced
from an NMR analysi's. The isolated y;eld of pure ~an~-a,~-unsaturated
aldehyde~ based on its precursor ~n~-dimethoxymethyl~ut-2-enen;trile, was 91%.
Example 5. ~an~-3-Hydroxymethylbut--2-enen;trile ~VIII)
To a solution of 3.2 g of ~an~-3-formyl-2-enenitrile in 80
ml of ethanol, cooled în an ice ba~h and sti'rred wi`th a magnetic stirrer,
wa~ added 1.28 9 of NaBH4 in port;ons over a period of 5 m;nutes. The
m~xture was then kept st;rred in the ;ce bath for an add;tional 10
minutes and subsequently decomposed w;th 100 ml water. The aqueous ethanol
solution was extracted four times wi't~ 100 ml portions of ether. The
combined ethereal extract ~as ~ashed with 200 ml of ~ater to remove most
of the ethanol. The orsanic phase ~as then dried and evaporated to re-
move the solvent to give practically pure ~an~-3-hydroxymethylbut-2-
enen;tr;le ~VIII~ (2.~4 9, ~0~ yield). An analytical sample purified
by dist;llat;on (72 - 73C/0.15 mm Hg, 68 - 71C/0.25 mm Hg) was identical
to that reported in the literature (Letham et al.).
Example 6. 3-Hydroxymethylbutylamine ~VII)
To a solution of 5 9 of ~han~-3-formyl-2-enell;trile in 100 ml
of methanol coaled in an ice ~ath and stirred with a magnetic stirrer, was
I ~726A4
added 2 g of NaBH4 over 5 minutes. After stirring for an additional 10
minutes in the ice bath, 12.6 9 of CoC12 6tI20 was added and left to stir
for 5 minutes. To the above mixture was added slowly 8.1 g of NaBH4
over a period of 15 minutes. When the addition was complete, the reaction
mixture was left to stir for 2 hours before decomposing ~ith 60 ml of 6 N
hydrochloric acid. The resultant mixture was then basified with sodium
hydroxide pellets to pH 12 and extracted four times with 150 ml portions
of methylene chloride. The combined extracts ~ere dried over K2C03
and evaporated to remove the solvent, giving 4.Q g of crude 3-hydroxy-
methylbutylamine (74% yield). The crude amino alcohol had an 60 HzNMR spectrum in CDC13 of ~O.g (doublet, J=6 Hz, 3H), ~1.5 (multiplet,
3H), ~2.7 (multiplet, 2H), ~3.4 (doublet, J=5.5 Hz, 2Hl and a D20
exchangeable singlet at around ~3.5 (3H). The product was sufficiently
pure for the synthesis of racemic dihydrozeatin by the method of
Fujii et al. An additional amount (430 mg~ of the crude amino alcohol
product, which was less pure than that isolated above, ~as o6tained by
continuous extraction of the aqueous solution with ether or methylene
chloride.
Example 7. ~a~-3-Hydroxymethyl-2-enenylami;ne sulfate (XI:~
The compound ~a~6-3-h~droxymethylbut-2-enenitrile
~-butyldimethylsilyl ether (X) was; prepared in quanti:tative yield
from ~an~-3-hydroxymethyl~ut-2-eneni~rile and ~h~-but~ldi:methylsilyl
chloride by the method oF E. J. Corey and A. Venkateswarlu, J. Amer. Chem.
Soc. 94, 6190, 1972. The compound X had the following physical parameters:
b.p. 72 - 74C/0005 mm Hg, NMR (80 MHz, TMS as an internal standard in
CDC13solvent) ~0.07 (singlet, 6H), ~Q.~2 (singlet, 9H), ~1.96 (doublet
of triplet, J = 1.1 Hz, J = 0.85 Hz, 3H)~ ~4.15 (doublet of quartets, J =
2 Hz, J = 0.85 Hz, 2H), ~5.53 (eight-line multiplet, J = 2 Hz, J = 1.1
Hz), IR (liquid film) 2955, 2930, 2220, 16809 1255, 1120, 840~ and 780 cm 1.
To a solution of 3 g oF the above nitrile (X) in 100 ml of
- 16 -
`~ 17~6~4
anhydrous ether, cooled and stirred in an ice bath, was added 810 mg of
LiAlH4 over 5 minutes. The mixture was stirred 1 hour in an ice bath,
1 hour at room temperature and 1 hour at reflux temperature. The
resultant complex was allowed to cool, was decomposed with 2.5 ml of
5 water and then treated with MgS04 to aid coagulation of the solid. The
solid was removed by filtration and washed thoroughly with 100 ml of
ether. After evaporating the solvent, 2.6 9 oF a light yellow liquid
product were obtained. Gas chromatographic analysis of the crude product
showed it to contain 70% of the desired unsaturated amino alcohol silyl
0 ether. A pure colorless sample obtained by distillation (64 - 67C/0.05
mm Hg) had an NMR spectrum (80 Hz in CDC13 solvent) of ~0.06 (~singlet, 6H~,
~0.93 (singlet, 9H), ~1.62 (singlet with fine splitting J = 1.4 Hz, 3H~,
ôl.84 (broad singlet, D20 exchangeable, 2tl), ~3.20. (doublet, J = 6.8 Hz
with fine splitting), ~4.05 (singlet with fine splitting, 2H~, ~5.56
15 (triplet of quartet, J = 6.8 Hz, J = 1.4 Hz, lH)i an IR (liqui:d fi:lml of
2955, 2q30, 2860, 1670, 1550, 1473, 1465, 134a, 1363, 1255,1a80, lOn5, 940,
835, and 773 cm 1.
A quantity of 1.8 9 of tfie above p~epared crude amino alcohol
was dissolved in 20 ml of methanolic s;ulfuri`c acid (5~ v~v lN H2S04 in
20 methanol) and stirred at room temperature ~or 4 hours. The resultant
mixture was washed twice with 30 ml porti:on~ o~ ether. The aqueous layer
was separated and evaporated under reduced pressure at 50C to yield 1.3 9
of a light bro~n syrup. The liquid product had an NMR spectru~
(80 Hz in D20) af ~1.74 (;singlet, 3H~, ~3.7 (doublet, J = 7.5 Hz, 2H~,
25 ~4.0 (singlet, 2H),ô5.5 (triplet, 7.5 Hz).
The sulfate (XI) could not be easily obtained in a crystalline
form, however, it can be used directly for the synthesis of zeatin
according to the method of G.Shaw et al.,J. Chem. Soc. (c), ~21, lq66.
- 17 -
26~
It should be realized that although the present invention has
been disclosed with an objective of providing a synthetic pathway to zeatin,
Gne skilled in the art will recogn;ze that the *~a~-3-formylbut-2-enenitrile
(V) product formed herein is a isoprenoid which may have further utility in
the synthesis of other natural products or useful chemical building blocks.
Furthermore~ while the present invention has been disclosed
in connection with the preferred embodiments. thereof, it should be under-
stood that there may be other embodiments whi:ch Fall within the spirit
and s.cope of the present i.nvention as. defi:ned by the follo~ing claims.
~ '
- 18 -
