A hetero diels-alder access to (Z)-zeatin and (Z)-isozeatin

Article abstract of DOI:10.1135/cccc19990696

(Z)-6-(4-Hydroxy-3-methylbut-2-en-1-ylamino)purine, (Z)-zeatin, and the isomeric (Z)-isozeatin, both free from the E isomers, were prepared using the cycloaddition of the in situ generated tert-butyl nitrosoformate on isoprene. A mixture of tert-butyl 5(and 4)-methyl-3,6-dihydro-2H-1,2-oxazine-2-carboxylates thus formed was deprotected and reductively cleaved to (Z)-4-amino-2(and 3)-methylbut-2-en-1-ols, which were chromatographically separated and finally alkylated with 6-chloropurine to give the title compounds.

Full text of DOI:10.1135/cccc19990696

696
Tolman, Hanuš, Sedmera:
A HETERO DIELS–ALDER ACCESS TO (Z)-ZEATIN AND (Z)-ISOZEATIN
a2
b
Vladimír TOLMANa1,*, Jan HANUŠ and Petr SEDMERA
a
Isotope Laboratory, Institute of Experimental Botany, Academy of Sciences of the Czech Republic,
Vídeňská 1083, 142 20 Prague 4, Czech Republic; e-mail: holik@biomed.cas.cz,
1
2
hanus@biomed.cas.cz
b
Institute of Microbiology, Academy of Sciences of the Czech Republic, Vídeňská 1083,
142 20 Prague 4, Czech Republic; e-mail: sedmera@biomed.cas.cz
Received Decem ber 17, 1998
Accepted February 3, 1999
(Z)-6-(4-Hydroxy-3-m eth ylbut-2-en -1-ylam in o)purin e, (Z)-zeatin , an d th e isom eric (Z)-iso-
zeatin , both free from th e E isom ers, were prepared usin g th e cycloaddition of th e in situ
gen erated tert-butyl n itrosoform ate on isopren e.
A m ixture of tert-butyl 5(an d
4)-m eth yl-3,6-dih ydro-2H-1,2-oxazin e-2-carboxylates th us form ed was deprotected an d
reductively cleaved to (Z)-4-am in o-2(an d 3)-m eth ylbut-2-en -1-ols, wh ich were ch rom ato-
graph ically separated an d fin ally alkylated with 6-ch loropurin e to give th e title com poun ds.
Key w ord s: Cytokin in s; Zeatin ; Nitrosoform ates; Dien e syn th esis; 3,4-Dih ydrooxazin es;
Cycloaddition s; Diels–Alder reaction ; Purin es.
Both E an d Z isom ers of zeatin , 6-[4-h ydroxy-3-m eth ylbut-2-en -1-yl)-
am in o]purin e are im portan t m em bers of th e large fam ily of n atural
cytokin in s. Wh ile th e ch em istry of (E)-zeatin (1a) h as been th orough ly
elaborated sin ce its isolation by Leth am an d coworkers in 1964 (ref.1) an d
th e com poun d is syn th etically available by several m eth ods2,3, (Z)-zeatin
(2a) is accessible on ly with difficulties. Com poun ds 1a an d 2a are prepared
by con den sation of 6-ch loropurin e with am in oalcoh ol 3a an d 4a, respec-
tively. Th e m ain problem associated with th e syn th esis of 2a is th e lack of a
sim ple preparative path way for 4a. So far, four m eth ods, differin g in reli-
ability an d/or practical applicability, h ave been suggested for th e purpose.
A stereoselective access to 4a, based on th e reductive cleavage of th e N–O
bon d in 5-m eth yl-3,6-dih ydro-2H-1,2-oxazin e (7a) appeared as early as
1971. In th is paper4, th e dih ydrooxazin e was prepared by a Diels–Alder re-
action of 1-ch loro-1-n itrosocycloh exan e an d isopren e, followed by
alcoh olysis of th e prim ary adduct, in th e sam e way as described for th e
cycloaddition of 1-ch loro-1-n itrosocycloh exan e an d butadien e5. Neith er
th e experim en tal details, n or th e yields of th e in term ediates were reported.
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Diels–Alder Reaction
697
Our n um erous experim en ts con firm ed wh at could be read between th e
lin es: th e cycloaddition was far from bein g a clean reaction an d th e overall
yield was very low. Moreover, th e fin al am in e 4a was con tam in ated with
th e isom eric 4b, in dicatin g in sufficien t stereoselectivity of th e Diels–Alder
reaction .
HO
X
X
NH
N
NH
N
Y
HO
Y
N
N
N
N
N
H
N
H
2a,2b
1a,1b
a, X = Me; Y = H
b, X = H; Y = Me
X
X
Y
NH
2
HO
Y
HO
4a-4c
Z
a, X = Me; Y = H; Z = NH
3a,3b
2
b, X = H; Y = Me; Z = NH
2
c, X = Me; Y = H; Z = NPhth
Sh ortly th ereafter, Corse an d Kuh n le6 claim ed th at th ey isolated th e
(Z)-ph th alim ido derivative 4c as a byproduct in th eir syn th esis of 1a. We
h ave reproduced th eir procedure twice, except th at ch rom atograph ic sepa-
ration of th e isom ers was realized at th e stage of th e free 3a. In th e secon d
separed substan ce th e couplin g J = 6.8 Hz between th e olefin ic m eth in e an d
oxym eth ylen e proton s in dicated a vicin al arran gem en t of th ese groups.
Sin ce both olefin ic m eth yl an d m eth in e exh ibited n uclear Overh auser ef-
fect (NOE) to both CH2N an d CH2O proton s, th is com poun d was n ot 4a
but th e oth er E isom er7 3b. Th is was m ost probably also th e case of a later
m odification 8, wh ich used th e sam e startin g m aterials an d in term ediates.
On ly m in ute am oun ts of 4a were isolated from th e reaction of m eth yl
(Z)-4-brom o-2-m eth ylbut-2-en oate with sodium azide, followed by LiAlH4
reduction of th e 4-azido ester9. Th e Horn er–Wadsworth –Em m on s reaction
in its Still–Gen n ari m odification was succesfully applied in th e h igh ly
stereoselective syn th esis of 2a by Eviden te et al.10. Th eir well-elaborated
five-step reaction path way gives good results an d is n owadays th e on ly use-
ful preparative m eth od for 2a. However, it is disqualified by drawbacks,
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698
Tolman, Hanuš, Sedmera:
such as un com m on reagen ts, labor-con sum in g colum n ch rom atograph y af-
ter each step an d scalin g-up problem s.
In th e lastin g n eed for a sim pler access to 2a, we turn ed back to th e
Diels–Alder strategy, but with an appropriate ch an ge of th e n itroso com po-
n en t. Th e relatively n ew11,12 m etastable n itrosoform ates 5 were th e com -
poun ds of ch oice, possessin g several advan tages: easy in situ preparation
from th e correspon din g N-h ydroxycarbam ates, clean reaction with isopren e
an d sm ooth con version of th e prim ary adducts
6 in to th e free
3,6-dih ydro-2H-1,2-oxazin es 7 (Sch em e 1).
O
O
X
N
O
OR
+
N
O
OR
Me
a, X = H; Y = Me; R = Bn
b, X = Me; Y = H; R = Bn
Y
5a, R = Bn
5b, R = Bu
6(a+b) or 6(c+d)
t
t
c, X = H; Y = Me; R = Bu
t
d, X = Me; Y = H; R = Bu
(i)
X
(ii)
NH
4a
4b
+
O
Y
a, X = H; Y = Me
b, X = Me; Y = H
7(a+b)
(i) TFA (for mixture 6c and 6d)
(ii) AlHg /MeOH, H O
x
2
SCHEME 1
Our in itial experim en ts were carried out with ben zyl N-h ydroxy-
carbam ate13, wh ich was oxidized, in th e presen ce of isopren e, with
tetrabutylam m on ium periodate14 to afford a m ixture of 6a an d 6b in a total
yield of 48%. Structures of 6a an d 6b were assign ed to th e products by
determ in in g wh ich m eth ylen e (CH2O or CH2N) exh ibited th e 6a/6b large
vicin al couplin g to th e olefin ic proton . NMR an alysis sh owed, h owever, th e
isom er ratio 30 : 70, i.e. on ly a low proportion of th e required 6a. Hopin g
th is ratio m ay be in fluen ced by steric factors, we n ext used th e m ore bulky
dien oph ile 5b an d obtain ed a m ixture of 6c an d 6d in 52% yield an d in a
ratio 55 : 45. Rem oval of th e ester groupin g by trifluoroacetic acid th en
gave a m ixture of dih ydrooxazin es 7a an d 7b (as trifluoroacetates), wh ich
was furth er reduced with out purification to th e crude am in o alcoh ols 4a
an d 4b. For th is reduction , alum in ium am algam was foun d preferable to
th e previously used zin c in acetic acid15. Th e reduction m ixture was
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Diels–Alder Reaction
699
ch rom atograph ically separated in to th e in dividual 4a an d 4b in th e yields
of 24.1 an d 25.6% (based on th e sum 6c an d 6d ), respectively. Both sepa-
rated 4a an d 4b were con verted in to th e crystallin e h em ioxalates, wh ich
were fin ally con den sed with 6-ch loropurin e to give th e desired (Z)-zeatin
(2a) an d, in addition , th e isom eric (Z)-isozeatin (2b). On ly on e referen ce to
2b h as been foun d in th e literature. Its syn th esis h as been m en tion ed as be-
in g “in press” (ref.8 in ref.16); h owever, th e paper n ever appeared.
EXPERIMENTAL
Meltin g poin ts are m easured on Büch i-SMP 20 an d are reported un corrected.
Ch rom atograph y was carried out on th e Merck silica gel 60 (230–400 m esh ) in ch loro-
form –m eth an ol–25% aqueous am m on ia: 40 : 6 : 1 (A), 40 : 10 : 2 (B), 70 : 30 : 5 (C), 40 :
10 : 1 (D). HPLC an alyses were perform ed on a reverse-ph ase colum n RP Select B (2 × 150
m m , 4 µm ; Merck) usin g a Beckm an n pum p (m odel 128) an d a Beckm an n PDA detector
(m odel 168). Th e solven t system was m eth an ol–40 m M am m on ium acetate buffer (pH 3.4);
its lin ear gradien t ch an ged from th e startin g ratio 20 : 80 to 35 : 65 (15 m in ) an d th en to
50 : 50 (10 m in ). NMR spectra were m easured on Varian VXR-400 or In ova-400 spectrom e-
ters (400 MHz for 1H, 100 MHz for 13C) at 25 or 30 °C in th e in dicated solven ts (δ in ppm ,
W an d J in Hz). Carbon sign al m ultiplicity was determ in ed by APT (Attach ed Proton Test).
COSY an d NOESY experim en ts used for th e assign m en t were perform ed usin g th e m an ufac-
turer’s software.
Ben zyl 5-Meth yl-3,6-dih ydro-2H-1,2-oxazin e-2-carboxylate (6a) an d
Ben zyl 4-Meth yl-3,6-dih ydro-2H-1,2-oxazin e-2-carboxylate (6b)
To a cool (–30 °C) solution of isopren e (1.02 g, 1.5 m l, 15 m m ol) an d ben zyl N-h ydroxy-
carbam ate13 (1.67 g, 10 m m ol) in ch loroform (70 m l), tetrabutylam m on ium periodate
(4.33 g, 10 m m ol) dissolved in ch loroform (30 m l) was added dropwise with in 45 m in . Th e
m ixture was stirred for 1.5 h , reach in g 0 °C an d for an oth er 1.5 h at room tem perature.
Th en it was wash ed with 10% aqueous sodium th iosulfate (4 × 15 m l), water (30 m l), an d
dried over an h ydrous sodium sulfate. Evaporation left
a red oil (3.9 g), wh ich was
ch rom atograph ed on silica gel (100 g, ch loroform ) to give 1.12 g (48%) of a m ixture of 6a
an d 6b as a colorless viscous oil. NMR sh owed th e ratio 6a : 6b = 30 : 70. 1H NMR (CDCl3,
30 °C): δ m ajor 1.739 (3 H, m , CH3-C=); 4.021 (2 H, m , W = 8.9, CH2N); 4.390 (2 H, m , W =
14.6, OCH2); 5.223 (2 H, s, CH2Ph ); 5.536 (1 H, m , =CH); 7.297–7.406 (5 H, m , Ph ); m in or
1.663 (3 H, m , CH3-C=); 4.110 (2 H, m , W = 12.9, CH2N); 4.282 (2 H, m , W = 8.2, OCH2);
5.218 (2 H, s, CH2Ph ); 5.524 (1 H, m , =CH); 7.297–7.406 (5 H, m , Ph ). 13C NMR (CDCl3,
25 °C): δ m ajor 19.66 q, 48.55 t, 67.68 t, 68.55 t, 118.02 d, 128.13 d, 128.23 d (2 C), 128.52 d
(2 C), 130.13 s, 136.09 s, 155.51 s; m in or 18.23 q, 44.76 t, 67.68 t, 71.65 t, 116.28 d,
128.13 d, 128.23 d (2 C), 128.52 d (2 C), 131.58 s, 136.09 s, 155.51 s.
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Tolman, Hanuš, Sedmera:
tert-Butyl 5-Meth yl-3,6-dih ydro-2H-1,2-oxazin e-2-carboxylate (6c) an d
tert-Butyl 4-Meth yl-3,6-dih ydro-2H-1,2-oxazin e-2-carboxylate (6d )
Reverse addition : To tetrabutylam m on ium periodate (4.33 g, 10 m m ol) in ch loroform
(60 m l) con tain in g m olecular sieve 4A (2.35 g),
a solution of tert-butyl N-h ydroxy-
carbam ate17 (1.33 g, 10 m m ol) an d isopren e (1.36 g, 20 m m ol) in ch loroform was added
dropwise at –30 °C durin g 1 h . Th e tem perature of th e m ixture was th en slowly raised to 0 °C
durin g 1.5 h an d stirrin g was con tin ued for an oth er 1.5 h with out coolin g. Th e solution was
con cen trated to a sm all volum e an d ch rom atograph ed in th e sam e way as in th e previous
paragraph . Th e fraction s con tain in g th e product were com bin ed, sh aken with solid sodium
th iosulfate (11 g) to rem ove pin k color, filtered an d evaporated. Yield 1.04 g (52%) of a
yellowish oil; accordin g to NMR, th e ratio of 6c : 6d was 55 : 45. 1H NMR (CDCl3, 30 °C): δ
m ajor 1.501 (9 H, s, t-Bu); 1.628 (3 H, m , CH3-C=); 4.030 (2 H, m , W = 14.1, CH2N); 4.256
(2 H, m , W = 10.9, OCH2); 5.516 (1 H, m , =CH); m in or 1.507 (9 H, s, t-Bu); 1.736 (3 H, m ,
CH3-C=); 3.941 (2 H, m , W = 10.9, CH2N); 4.361 (2 H, m , W = 13.8, OCH2), 5.531 (1 H, m ,
=CH). 13C NMR (CDCl3, 25 °C): δ m ajor 18.75 q, 28.85 q (3 C), 44.81 t, 71.16 t, 81.48 s,
118.09 s, 155.63 s (C=O); m in or 19.75 q, 28.83 q (3 C), 48.62 t, 68.00 t, 81.48 s, 116.59 d,
130.38 s, 155.63 s.
(Z-)-4-Am in o-2-m eth ylbut-2-en -1-ol (4a) an d (Z)-4-Am in o-3-m eth ylbut-2-en -1-ol (4b)
A m ixture of 6c an d 6d , obtain ed as above (2.03 g, 10.2 m m ol) was dissolved in dich loro-
m eth an e (5 m l) an d trifluoroacetic acid (6.5 m l) was added. Th e solution was set aside for
1 h at room tem perature, evaporated in vacuo an d repeatedly evaporated with m eth an ol (2 ×
30 m l). Th e resultin g oily m ixture of 7a an d 7b (as trifluoroacetates) was dissolved in water
(4.7 m l) an d added to am algam ated alum in ium foil (1.81 g, 67 m m ol) in m eth an ol (70 m l).
Th e m ixture was stirred vigorously for 3 h at am bien t tem perature, filtered, th e in organ ic
m atter was th orough ly wash ed with m eth an ol an d th e com bin ed filtrates evaporated to give
a yellow oil (1.80 g). Th is was ch rom atograph ed on silica gel (120 g); solven t A, followed by
B eluted 4b , wh ereas for 4a solven t C was used. Th e in dividual am in oalcoh ols were
rech rom atograph ed on a sm aller silica gel colum n (50 g) in solven t B. Th e yields of pure
com poun ds were: 4a, 249 m g (24.1%), yellowish oil; 4b, 264 m g (25.8%), yellow oil, crystal-
lizes in th e freezer. In addition to pure isom ers, 75 m g (7.3%) of an un separated m ixture 4a
an d 4b was obtain ed.
4a: 1H NMR (CDCl3, 25 °C): δ 1.732 (3 H, dt, J = 1.5, 0.8, CH3-C=); 3.280 (2 H, dq, J = 7.1,
0.8, CH2N); 3.576 (3 H, br s, OH + NH2); 4.001 (2 H, br s, OCH2); 5.357 (1 H, tqt, J = 7.2,
1.5, 0.8, =CH). 13C NMR (CDCl3, 25 °C): δ 21.75 q, 38.06 t, 61.04 t, 125.53 d, 139.22 s.
4b: 1H NMR (CDCl3, 25 °C): δ 1.766 (3 H, dt, J = 1.5, 0.9, CH3-C=); 3.275 (2 H, br s,
CH2N); 4.036 (2 H, dq, J = 7.0, 0.9, OCH2); 5.544 (1 H, tqt, J = 7.0, 1.5, 0.9, =CH). 13C NMR
(CDCl3, 25 °C): δ 22.24 q, 41.53 t, 57.79 t, 126.98 d, 138.06 s.
Th e free bases 4a an d 4b were con verted in to h em ioxalates accordin g to ref.10
.
4a·0.5 (CO2H)2: yield 306 m g (85%), m .p. 182–184 °C (dec.), (ref.10, m .p. 183.5–185 °C).
1H NMR (D2O, 30 °C): δ 1.848 (3 H, dt, J = 1.1, 0.75, CH3-C=); 3.686 (2 H, dq, J = 7.5, 1.1,
CH2N); 4.163 (2 H, d, J = 0.75, OCH2); 5.466 (1 H, ttq, J = 7.5, 1.1, 0.75). 13C NMR (D2O,
30 °C): δ 23.48 q, 39.28 t, 62.76 t, 121.01 d, 145.64 s, 176.28 s (C=O).
4b ·0.5 (CO2H)2: yield 315 m g (82.5%), m .p. 162–165 °C (dec.); from water–eth an ol
(1 : 10), m .p. 168–170 °C (dec.). For C6H12NO3 (146.2) calculated: 49.30% C, 8.28% H,
9.58% N; foun d: 49.25% C, 8.41% H, 9.50% N. 1H NMR (D2O, 30 °C): δ 1.865 (3 H, dt, J =
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Diels–Alder Reaction
701
1.6, 0.8, CH3-C=); 3.685 (2 H, d, J = 0.8, CH2N); 4.164 (2 H, dq, J = 6.0, 1,1, OCH2); 5.785
(1 H, ttq, J = 6.0, 1.1, 1.6, =CH). 13C NMR (D2O, 30 °C): δ 23.43 q, 42.07 t, 59.96 t, 133.14 d,
133.78 s, 176.23 s (C=O).
(Z)-Zeatin (2a)
Th e suspen sion of 4a·0.5 (CO2H)2 (146 m g, 1 m m ol) an d 6-ch loropurin e (232 m g, 1.5
m m ol) in propan -1-ol (5 m l) con tain in g trieth ylam in e (405 m g, 4 m m ol) was stirred un der
argon for 5 h at 90 °C. Th e m ixture was evaporated in vacuo an d ch rom atograph ed on a silica
gel colum n (3 × 14 cm ) in system D, givin g 2a (189 m g; 80.6%, based on th e startin g oxa-
late), m .p. 212–214 °C; after recrystallization from eth an ol, m .p. 214–215 °C (m .p. in ref.4,
206–208 °C; in ref.10, 212–214 °C). Th e HPLC purity was over 99%. 1H NMR (DMSO-d6,
30 °C): δ 1.703 (3 H, bs, =C-CH3); 4.036 (2 H, J = 5.2, CH2O); 4.131 (2 H, m , NCH2); 4.755
(1 H, t, J = 5.2, OH); 5.347 (1 H, t, J = 6.8, =C-H); 7.565 (1 H, bs, NH); 8.064 an d 8.160 (1 H
each , s, purin e proton s).
(Z)-Isozeatin (2b)
A suspen sion of 4b·0.5 (CO2H)2 (44 m g, 0.3 m m ol) an d 6-ch loropurin e (30.8 m g, 0.2 m m ol)
in propan -1-ol (2 m l) con tain in g trieth ylam in e (141 m g, 1.4 m m ol) was stirred for 12 h at
90 °C, evaporated, dissolved in 50% m eth an ol an d applied on a sm all colum n of Dowex
50 × 8 in [H]+ cycle. Th e product, togeth er with un reacted 4b, was eluted with 1 : 1 m ixture
of m eth an ol–3% aqueous am m on ia an d recrystallized from eth an ol (1 m l). Yield, 29 m g
(66%, calculated on 6-ch loropurin e, 44% on 4b), m .p. 216–217 °C. HPLC purity was better
th an 99%. For C10H13N5O (219.2) calculated: 54.78% C, 5.98% H, 31.95% N; foun d: 54.67% C,
6.08% H, 32.11% N. 1H NMR (DMSO-d6, 30 °C): δ 1.684 (3 H, dt, J = 1.2, 1.2, =C-CH3);
3.417 (2 H, bs, NCH2); 4.085 (2 H, d, J = 6.6, OCH2); 4.147 (1 H, bs, NH); 5.424 (1 H, ttq, J =
6.6, 1.0, 1.2, =C-H); 8.088 an d 8.161 (1 H each , s, purin e proton s).
This work was done under support from the grant No. 204/96/0391, given by the Grant Agency of
the Czech Republic.
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Tolman, Hanuš, Sedmera:
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