Syntheses of Some Proposed Biosynthetic Precursors to the Isocyclic Ring in Chlorophyll a

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In connection with studies on the biosynthesis of the isocyclic ring E in chlorophyll a (1), the magnesium(II) 2,4-divinylporphyrin β-keto ester 8 and hydroxypropionate 6 have been synthesized. The key porphyrin intermediate 36 in these syntheses was obtained most efficiently by cyclization of a 1’-methyl-8’-unsubstituted a,c-biladiene dihydrobromide 31 in hot o-dichlorobenzene. An alternative route through copper(II)-promoted cyclization of a 1’,8’-dimethyl-a,c-biladiene hydrobromide, 29, gave lower yields of porphyrin. The keto ester groups were added by way of the corresponding porphyrin imidazolides 20b and 39 by using the magnesium(II) complex of methyl hydrogen malonate. Borohydride reduction of the magnesium(II) 2,4-divinylporphyrin β-keto ester 8 gave the corresponding hydroxypropionate porphyrin 6, but attempts to dehydrate this to give the acrylate 12, an intermediate also of possible biological significance, were unsuccessful. © 1986, American Chemical Society. All rights reserved.

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Journal of Organic Chemistry

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