Alkyl-5,6-dihydro-a-pyrones

Several syntheses of parasorbic acid (7) have been reported (6-15). Osmundalactone (8) has also been isolated from the fungus Paxillus atrotomentosus (16). A synthesis of chiral aspyrone (9) has confirmed its absolute stereochemistry (17) and studies on its biosynthesis have continued (18). 

In the same year Tsuda et al. (27) reported the isolation of cytotoxic sesterterpenes from the Okinawan marine sponge Luffariella sp. among which was a compound with a 5,6-dihydro-a-pyrone moiety, luffariolide E. This compound showed cytotoxicity against murine leukaemia LI210 cells (IC50 1.1-7.8 pg/ml) in vitro. The original structure, which rested on chemical and spectroscopic evidence, has been corrected by synthesis to (24) (28). 

Both the natural (—) and unnatural (+) enantiomers of the antifungal podoblastins A, B and C (25-27) and of synthetic podoblastin S (28) have been synthesized from the corresponding R(—)- and S(+)-glycerol derivatives (29). The (6S)-isomers were considerably less active against rice blast than the (6R)-isomers. 

Further syntheses of (-)-tarchonanthus lactone (29) have appeared (30, 31). Several syntheses of racemic and enantiomerically pure massoialactone (30) have been reported (9, 77, 32-37). Pestalotin (31) and hydroxypestalotin (LL-PSSOP) (32) have been isolated from Pestalotiopsis oenotherae (38). Syntheses of natural and racemic pestalotin as well as the (67 )-isomer have been published (39-41). All 

Syndenolide (35) together with the known 5,6-dihydro-a-pyrone, deacetylboronolide (36), was isolated from Syncolostemon densiflorus (Lamiaceae) 49). The presence of a triol in (35) was demonstrated by D2O induced collapse of three hydroxyl proton signals in its NMR 

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