Synthetic Photochemistry. XXXV. A Synthesis of Bis (dehydroxy) xanthocidin Methyl Ester from the Partially Optical Active Photoisomer of β-Cyclodextrin-included 2-Methoxytropone

Abstract

Starting from the partially-optical active 7-methoxybicyclo[3.2.0]hepta-3,6-dien-2-one, a photoisomer of 2-methoxytropone obtained from the B-cyclodextrin inclusion complex, bis(dehydroxy)xanthocidin methyl ester was synthesized. In 1978, we found2' a photoisomerization of troponoids under a-and P-cyclodextrin (CDX)-included conditions to give bicyclo(3.2.0]hepta-3,6-dien-2-ones with a remarkable rate enhancement and with much improved yields for proto-photoisomers in various dispersion media. The photoproducts derived from tropolone (la) and 2-methoxytropone (lb) were optically active, and by using these optically active specimens, we have carried out a mechanistic investigation of the "antara-antara"-3, 3-sigmatropy;3' the thermolysis of optically active 1-methoxybicyclo[3.2.0]hepta-3,6-dien-2-one (2) to optically active 3-methoxybicyclo[ 3.2.0 ]hepta-3, 6-dien-2-one (3), ruled out an involvement of radical intermediate.`•5•6) Obviously, the photoproduct (2) is good starting material for polysubstituted cyclopentane derivatives, and by this mean we have now synthesized a dialkyl derivative (4) of sarkomycin (B)7' which can be, in same time, designated as a bis(dehydroxyl) derivative of xanthocidin8' from 7-methoxybicyclo[3.2.0Ihepta-3, 6-dien-2-one (5). The findings will be described in this paper. An a-methylenecyclopentanone antibiotic, xanthocidin (A) was first isolated from a Streptomlces species, IVo.51--4by Asahi et al.8' and its racemic form has been synthesized by Smith9' via the key intermediate, bis(dehydroxy)-4, 5-dehydroxanthocidin (C). The above mentioned readines$ in furnishing the cyclopentene synthons prompted us to carry out the experiments on the synthesis of this methylene cyclopentanone relative, methyl ester of bis(dehydroxyl) derivative (4) of A'O' as follows. Prolonged irradiation of lb under the B-CDX-inclusion conditions in cyclohexane afforded optically active 5'i' via the proto-photoisomer (2),i2' 1-methoxyl isomer, in 470/o yield with 550/o enantiomeric excess. Optical activities of these compounds were determined by derivation to 3oxocyclopentaneacetic acid (6). By the Grignard reaction of 5 with isopropylmagnesium bromide and subsequent quenching of the resultant magnesium enolate with methyl iodide, 4-isopropyl-7-methoxy-3-methylbicyclo[3.2.0]hept-6-en-2-one (7) was obtained in 630/o yield in a one-pot operation. The direction of the attack of bulky isopropyl group to the *Department of Molecular Engineering -20Partial Asymmertric Synthesis of Xanthocidin Derivatives from Troponoids