[formula: see text] A new approach to a suitably functionalized tricyclic core of sarains has been developed by means of Katritzky's cycloaddition using 3-oxidopyridinium betaines. A key step was the regioselective differentiation of the two nearly identical hydroxy groups derived from oxidative cleavage of the double bond in 8 to afford 14. A stereocontrolled construction of the tricyclic core 20 of sarains containing the requisite side chain at C-3' was achieved by an intramolecular conjugate addition.
Starting with the tricyclic core 2b, annulation to form the 13-membered western ring of sarain A has been achieved to afford the macrocycle 30a by initial construction of the sterically congested quaternary center at C-3, followed by elaboration of the C-3 side-chain and ring-closing olefin metathesis. Also included is a parallel conversion of tricycle 2c to macrocycle 30b containing a functionalized side-chain at N-1 suitable for attachment of the eastern macrocyclic ring.
[reaction: see text] An eight-membered ring was conveniently appended onto a cycloalkene carboxylate by employing a facile radical cyclization-fragmentation reaction of an omega-bromoalkyl spirocyclobutanone, which was readily accessible by the Kulinkovich cyclopropanation and subsequent electrophilic addition to a 3-bromoalkyl acetal.
[reaction: see text]. Intermolecular and intramolecular [4 + 3] cycloaddition reactions of readily available cyclopropanone hemiacetals with furans are described.
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