Iodolactonization oxidative cleavage

A cursory inspection of key intermediate 8 (see Scheme 1) reveals that it possesses both vicinal and remote stereochemical relationships. To cope with the stereochemical challenge posed by this intermediate and to enhance overall efficiency, a convergent approach featuring the union of optically active intermediates 18 and 19 was adopted. Scheme 5a illustrates the synthesis of intermediate 18. Thus, oxidative cleavage of the trisubstituted olefin of (/ )-citronellic acid benzyl ester (28) with ozone, followed by oxidative workup with Jones reagent, affords a carboxylic acid which can be oxidatively decarboxylated to 29 with lead tetraacetate and copper(n) acetate. Saponification of the benzyl ester in 29 with potassium hydroxide provides an unsaturated carboxylic acid which undergoes smooth conversion to trans iodolactone 30 on treatment with iodine in acetonitrile at -15 °C (89% yield from 29).24 The diastereoselectivity of the thermodynamically controlled iodolacto-nization reaction is approximately 20 1 in favor of the more stable trans iodolactone 30. 

Diketo diester 512 has played a key role in one of Paquette s approaches to the pentagonal dodecahedrane. Direct hydroboration-oxidation of502 provided as the principal product the unwanted isomer 511 (49 %) rather than 512 (30 %). This complication was circumvented by the improvisation of the cross-corner oxygenation sequence outlined in Scheme 75.420,42 The iodolactonization of diacid 510 proceeded with high efficiency to give 513 which underwent cleavage to 514 in the presence of methanolic sodium methoxide at room temperature. Oxidation to a-iodo ketone 515 followed by reductive deiodination with zinc-copper couple and ammonium chloride in methanol solution furnished isomerically pure 512. 

Ring cleavage. Several oxidants can cleave lactols at the Ca—C 3 bond, and the PhI(OAc)2-l2 pair is useful for attaching an iodine atom to C(3 in the process. From bicychc lactols such as those prepared from alkylation of cycloalkanones with epoxides as substrates iodolactones are obtained. ... 

---