Preparation of (3S)-N,N-1,7-Bis-tert- butoxycarbonyl-3-hydroxyspermidine in High Enantiomeric Purity

Total synthesis of (-)-crambidine and definition of the relative configuration of its unique tetracyclic guanidinium core

Total syntheses of the 3S,8S,10S,19R,43S isomer 4a and the 3S,8S,10S,19R,43R isomer 4b of the unique crambescidin alkaloid crambidine are reported. These studies confirm the tetracyclic structure proposed by Braekman and co-workers for crambidine, and establish the rel-3R,8R,10R,19S relative configuration for this moiety. Natural crambidine is most likely the 3S,8S,10S,19R,43S isomer 4a. These syntheses were completed in five steps and approximately 14% overall yield from 1-iminohexahydro[1,2-c]pyrimidine carboxylic ester 10, an intermediate in our earlier total synthesis of 13,14,15-isocrambescidin 800 (3). The signature step in the total syntheses of crambidine and several stereoisomers is chemoselective dehydrogenation of the tethered Biginelli adduct 10 or the derived tetracyclic intermediate 17. Additionally, these studies reveal the unprecedented ring-chain isomerization of the crambidine ring system exemplified by the interconversion of isomers 15a and 15b.

Synthesis of polycyclic guanidines by cyclocondensation reactions of N-amidinyliminium ions

A new method for the synthesis of polycyclic guanidines is described. The N-amidinyliminium ion generated from alpha-(phenylthio)amidine precursor 16 by reaction with Cu(OTf)(2) undergoes cyclocondensation with 1,3-dienes, styrenes, and beta-dicarbonyl compounds to give 1-iminohexahydropyrrolo[1,2-c]pyrimidines having side chains at C3 and C7. In all cases, major products have a cis relationship of the C7 side chain and angular C4a hydrogen, whereas C3 side chains are incorporated with lower stereoselectivity (dr = 2--5:1) in cyclocondensations with dienes and styrenes to give stereoisomer 39 as the major product. In contrast to most cycloadditions of alkenes with N-acyliminium ions, cyclocondensations of alkenes with N-amidinyliminium ions proceed by a stepwise pathway. Cyclocondensation of the cognate ureido aminal 31 with styrene provides the rare 2-imino-5,6-dihydro-4H-1,3-oxazine derivative 32, rather than a pyrimidine as the major product. The high stereoselectivity observed in condensations of 16 with benzyl acetoacetate to afford Biginelli adduct 29 supports the intermediacy of N-amidinyliminium ions in related tethered Biginelli condensations of guanidines reported earlier from our laboratories.