YO1 5DD X-Ray analysis of secalonic acid A (C,2H,,014,2CH,C02H) has confirmed the 2.2'-nature of the biphenyl linkage between the two halves of the molecule. Together with the known stereochemistry at C(6). C(6') (I), this provides the absolute configuration of the molecule. The analysis has also indicated how the acetic acid molecules are incorporated in the crystal structure by hydrogen bonding. Estimation by n.m.r. of the torsion angle between adjacent C-OH and C-Me groups in secalonic acid Afrorn solvent-induced changes of chemical shift was confirmed by the X-ray results.
Peracetic acid oxidation a t -78 "C of the dihydroxybicyclo[3.2.0] heptan-6-one (23) afforded the dihydroxylactone (24) which was protected as its bisdimethyl-t-butylsilyl ether (26) and reduced to the corresponding lactol (27). A Wittig reaction on ( 27), carried out in benzene with a short reaction time, gave mainly the required 11 ccsilyl ether (28) together with a trace of the 9a-silyl ether (29) which results from 1,5-migration of the silyl group.Oxidation of (28) followed by quantitative deprotection using aqueous H F in acetonitrile afforded (f) methyl ester (20). This short stereo-and regio-selective total synthesis proceeds in an overall yield of 10% starting from cyclopentadiene.
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