J . Org.Chem. 1 9 8 3 , 4 8 , 2298-2300
2298
The first event in the diagenesis of proteins in fossils is likely the cleavage of labile peptide bonds,14i.e., those involving sterically unhindered residues, and hydroxy and acidic amino acid residues. After these initial cleavages, peptide decomposition should proceed largely by successive formation of diketopiperazines according to eq 1. The liberated diketopiperazines then undergo hydrolysis to dipeptides, which are in turn hydrolyzed to free amino acids. Rapid racemization occurs in the diketopiperazine intermediate.3 The subsequent hydrolysis of the diketopiperazines into dipeptides and eventually free amino acids would thus yield the highly racemized low molecular weight peptides and free amino acids that are observed in calcareous f o s ~ i l s . ~ J ~ J ~ In addition to the geochemical implications, the diketopiperazine hydrolysis mechanism for peptides may also have implications for the thermal processing of food proteins, where the rapid rate of racemization accompanying diketopiperazine formation could produce significant amounts of amino acids. Several diketopiperazines have been identified in aged and heated foods and in protein hydrolysates.16 The presence of D-aminO acids in foods has important nutritional consequence^.'^
berellins through total synthesis has recently been established4 for the C19compounds, e.g., gibberellic acid, but only one of the simplest CZO gibberellins, (*)-GA15 (l),has 20
O-CH2
O-CH2
HO
2
chart I
4
3
/
(1) This work was described at the Seventh National Convention of the R.A.C.I., Canberra, Australia, Aug 1982. (2) Queen Elizabeth I1 Fellow.
EH3
6 Chart I1 u
Y
0.
HO
I1
A New Strategy for CzoGibberellin Synthesis: Total
Sir: The molecular basis for the biological activity of the gibberellin phytohormones has yet to be established, while many questions relating to their biosynthesis also remain unan~wered.~Reasonably practical access to the gib-
EOzCH3
5
Spencer M.Steinberg, Jeffrey L. Bada* Amino Acid Dating Laboratory Scripps Institution of Oceanography University of California, San Diego La Jolla, California 92093 Received February 9,1983
--
16
,,cH~
been prepared by total synthesis, and then from an arduous 45-step s e q u e n ~ e . ~
CH3
Summary: The extension of the strategy used for C19 gibberellin synthesis to include the CZOgibberellins has been realized through elaboration of the common intermediate 7 to give aldehyde 10 and then a novel intramolecular conjugate addition of an anionic ester species via a six-membered transition state, 3 4, followed by the intramolecular aldol cyclization, 5 6.
l4
15
' ~ 0 2 ~
I
(13) Wehmiller, J. F.; Hare, P. E. Science (Washington, D.C.)1971, 173,907. Bada, J. L.; Schroeder, R. A. Earth Planet. Sci. Lett. 1972,15, 1. Schroeder, R. A.; Bada, J. L. Earth-Sci. Rev. 1976,12, 347. (14) van Kleef, F. S. M.; De Jong, W. W.; Hoenders, H. J. Nature (London) 1975,258, 264. (15) Kriausakul, N.; Mitterer, R. M. Geochim. Cosmochim. Acta 1980, 44,753. (16) Pickenhagen, W.; Dietrich, P. Helu. Chim. Acta 1973, 4 , 1078. Shiba, T.; Nunami, N. Tetrahedron Lett. 1974,6, 509. Takahashi, K.; Tadenuma, M.; Kitamota, K.; Sato, S. Agric. B i d . Chem. 1974,38,927. (17) Masters, P. M.; Friedman, M. Agr. Food Chem. 1979, 27, 507.
Methyl Ester'!'
4'
H I8CH3
Acknowledgment. This work was supported in part by a grant from the Alfred P. Sloan Foundation. We thank R. Doolittle for synthesizing the hexapeptide. Registry No. Leu-Gly-Gly, 1187-50-4;Gly-Leu-Gly,2576-67-2; Phe-Gly-Leu-Gly-Val-Gly, 85864-65-9;c-(-Leu-Gly-),5845-67-0; c-(-Gly-Gly-), 106-57-0;c-(-Phe-Gly-),10125-07-2;c-(-Val-Gly-), 16944-60-8.
Synthesis of (*)-Gibberellin
H
Reagents: " B H ~ . D M S ,C r O 8 . 2 P y . b Z n . TiCI,, NOzHPO.. 'NOBH..
10 CHzBr2. CTharylborane; H,O~,
d C r 0 s . 2 P y . ' L O A ; CHn=CHCH2Er, HMPA. ' K H , P h p S e 2 . h H p O e . 2,6-lutidine.
(3) Hedden, P.; Phinney, B. 0. ACS Symp. Ser. No. 3, 1979, 19,57. (4) (a) Corey, E. J.; Danheiser, R. L.; Chandrasekaran, S.; Keck, G. E.; Gopalan, B.; Larsen, S. D.; Siret, P.; Gras, J. L. J. Am. Chem. SOC.1978, 100,8034-8036. (b) Corey, E. J.; Danheiser, R. L.; Chandrasekaran, S.; Sinet, P.; Keck, G. E.; Gras, J. L. Ibid. 1978,100, 8031-8034. (c) Lombardo, L.; Mander, L. N.; Turner, J. V. Ibid. 1980,102,6626-6628. (d) Cossey, A. L.; Lombardo, L.; Mander, L. N. Tetrahedron Lett. 1980,21, 4383-4386. (e) Hook, J. M.; Mander, L. N., Urech, R. J.Am. Chem. SOC. 1980,102,6628-6629. (5) (a) Nagata, W.; Wakabayashi, T.; Narisada, M.; Hayase, Y.; Kamate, S. J. Am. Chem. Soc. 1971,93.5740-6758. Totally svnthetic routes
via relays, but lacking optical rklutions, have been e&biished for other C, gibberellins including (b) gibberellins AI6 and A3, (-46 steps) [Fujita, E.; Node, M.; Hori, H. J. Chem. SOC.,Perkin Trans.1 1977,611-621] and (c) gibberellin A,, (-35 steps) [Mori, K.; Takemoto, I.; Mataui, M. Tetrahedron 1976, 32, 1497-15021.
0022-326318311948-2298$01.50/0 0 1983 American Chemical Society
Communications
J. Org. Chem., Vol. 48, No.
In this communication we. report the development of a much more efficient approach to CzOgibberellin total synthesis based on an adaptation of our earlier strateg3pcd for the preparation of the C19analogues. The success of this present study, which was focused on the construction of the challenging GA,, structure 2: depended critically on the intramolecular Michael and aldol reactions (3 4 and 5 6, respectively) outlined in Chart I. The most difficult stage of the synthesis, however, proved to be the elaboration of the quaternary center at pro-C(lO)' (Chart 11)* Olefinic ester 7" was converted smoothly (BH3.Me2S; Cr03.2py,8 0 "C, 70% yield)g into ketone 8, mp 149-151 "C,l0 but homologation of the new carbonyl function proved to be difficult. All standard reagents examined either failed to add to 8 or led to loss of stereochemical integrity at pro-C(9) because of enolization processes. A modification" of the Oshima methylenation procedure (Zn, TiC14, CH2Br2),however, furnished a 90% yield of the methylene derivative from which the loa-hydroxymethyl derivative 9, mp 180-182 "C, was obtained by hydroboration (thexylborane,12ether, 0 "C, 2 h; Na2HP04,H202, 35 "C, 3 h, 81% yield). Further oxidation (Cr03.2py, CH2C12,8 "C, 91% yield) to the aldehyde, mp 115-117 "C, followed by deprotonation (lithium diisopropylamide, THF, -90 "C, 5 min), and stereoselective alkylation on the more exposed 8-face with allyl bromide (HMPA, 18 "C, 5 h, 85% yield13) provided the adduct 10 as a homogeneous gum. This stage of the synthesis was completed by means of ~elenenylation'~ of 10 (KH, THF, Ph2&, 20 "C, 1.5 h)* at pro-C(6), followed by reduction of the formyl group (NaBH4,EtOH, 0 "C, 15 min) and selenoxide elimination (HzO2,ether, 2,6-lutidine, 20 "C) to afford the olefinic ester 11, mp 102-103 "C, in 85% overall yield. The lactone and A-ring moieties were then assembled smoothly as outlined in Chart I, although the annulation processes were not as facile as in the Clg series. Thus,ester 11 was converted to the propionate 3 [(EtCO),O, Et3N, 4-(dimethylamino)pyridine,room temperature, 1h, 95% yield], which was cyclized [KH, DMF, 2 h, -20 "C] under kinetic contro1,15and the resultant anion was quenched [Et3NH+OAc-,-40 "C] to give an -4:l mixture of &lactones 4 (100% crude yield), major isomerle crystallized, mp
-
-
(6) Hiraga, K.; Yokota, T.; Murofushi, N.; Takahashi, N. Agr. B i d . Chem. 1972,36(2), 345-347. (7) The atoms of all the Structures presented have been numbered on the basis of the full gibberellin skeleton: Rowe, J. R., Ed. "The Common and Systematic Nomenclature of Cyclic Diterpenes", 3rd rev.; Forest Product Laboratory, U.S.Dept. of Agriculture: Wisconsin, 1968. (8) Collins, J. C.; Hess, W. W.; Frank, F. J. Tetrahedron Lett. 1968, 3363-3366. (9) T h i ~ direct procedure was adapted from Gundu Rao et al. [Gundu Rao, C.; Kulpami, S. U.; Brown, H. C. J. Organomet. Chem. 1979,172, C2O-C22]; it was found that Collins reagent at 0 "C oxidized the inter-
mediate borane to the carbonyl compound. However, a higher overall yield (82%) was obtained from the stepwise procedure via the carbinol. (10) The structures of all new compounds are fully consistent with their NMR, IR, and mass spectral data. Satisfactory microanalyses (*
