Total syntheses of the ethyl esters of the host-selective (H-S) toxins produced b y Alternaria alternata (strawberry-type pathogen), AF-lla and AF-llc, and b y the Japanese pear-type AK-II are reported. The initial synthetic objectives, the 2€,4€,6€, 2€,4Z,6€ and 2€,4€,6Z stereoisomers of ethyl 8hydroxy-9-met hyldeca -2,4,6,9tetraenoate, were attained by the use of acetylene hydrometal lation and Pdo-mediated vinyl halide coupling: for this purpose tin chemistry was superior to the use of zirconium compounds. The tetraene-hydroxy esters were epoxidised selectively at the 9,lO-dou ble bond b y the Sharpless procedure under conditions of kinetic control (50% reaction), which it was predicted would lead to products of predominantly (8R,9S) -stereochemistry. Esterification of the epoxides produced from the 2€,4€,6€ and 2€,4€,6Z hydroxy esters w i t h synthetic (2R,3S) -2-(tbutyldimethylsiloxy) -3-methylpentanoic acid, followed by H PLC separation, gave as the major products, stereoisomers which, after deprotection, were characterised as the ethyl esters AF-toxin I Ic and AF-toxin lla. The stereochemical nature of the minor products in the epoxidations is considered. For the synthesis of the AK-I I toxin as its ethyl ester, similar esterification w i t h N-acetyl-L-phenylalanine was carried out, except that racemisation of the amino acid centre ensued. This led to two major products which were separated and identified as (8R,9Sr2'S) -and ( 8Rr9S,2'R) -stereoisomers, the former being identical with the ester of AK-II toxin.* Added in prooJ: AF-I and AF-111 toxins have E / Z patterns similar to AF-I1 toxins but with the 2'-hydroxy esterified with 2,3-dihydroxy-3methylbutyric acid and 2-hydroxy-3-methylbutyric acid respectively.8b

Syntheses of the (€)-I and ( 2 ) -2 forms of 1 -(3-methylbut-2-enyl)-3-{4-[3-(3,4-dimethoxyphenyl) -prop-2-enamido] butyl}urea, proposed structures for the natural hypotensive caracasanamide, have been carried out. The compounds were found not to be identical with the natural products, the structures of which have now been revised from containing a urea to containing a corresponding guanidine segment. Using aminoiminomethanesulfonic acid or 3-methylbut-2-enylcyanamide as reagents, the synthesis can be diverted to caracasanamide 15 (R = H) through an amine intermediate.Extracts of Verbesina caracasana (Compositae) show strong hypotensive effects in mice and the active principle (caracasanamide) has been reported to be the ( E )and (2)-forms of an amido urea 1 and 2.' As part of a wider ranging interest in the biological activities of natural amides,2 we report the synthesis of these two compounds and their relationship to revised guanidino structures which have since emerged.

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