Heterocyclic Synthesis and Novel Ring Systems

One of the earliest uses for rhodium(II)-catalyzed dipoles was demonstrated in Davies furan synthesis [22]. Isomiinchnones were also shown to produce substituted furans [115]. Additional furan syntheses have been described using silylacetates [116], unsaturated esters [117], and fluoroalkyl diazo acetates [118]. The synthesis of furofuranones and indenofuranones 35 from a-diazo ketones having pendant alkynes has also been reported (Eq. 6) [119]. Other fused heterocyclic systems include furo[3,4-c]furans [120, 121] furo[2,3-b]furans [122] as well as thiobenzofurans [123], and benzoxazoles[124] have also been synthesized with this methodology. 

Additional heterocyclic ring systems, such as benzofurans [125], dihydropyrroles and dihydroazepines [41], piperidines and dihydropyrimidines 36 [126], and fused oxazole derivatives [127], have been described (Eq. 7). The formation of epoxides and aziri-dines, formally emanating from ylides, was recently reported by Doyle et al. [77]. Rho-dium(II)-catalyzed isomiinchnone cycioaddition followed by Lewis acid-mediated ring opening has been used as an entry into the protoberberine azapolycyclic ring structure [128]. 

42 [44, 50] have been reported. Other complex ring systems have also been described, such as oxathioles and oxazolones [130], spiro-oxo molecules [129], oxatricyclic compounds [131], and spiroxindole like structures [132-135]. The synthesis of fused ring structures 

43 and 44 via the cycloaddition with quinone and quinone-like dipolarophiles was independently described by Pirrung [75, 136, 137], Muthusamy [138, 139], and Nair [140]. Stereoselective studies have generated syn-facial bridged norbomane structures [141, 142]. 

4) has been reported [143]. Interesting substrate-based selectivity was demonstrated with the dipolarophiles 45 and 47 in the carbonyl yhde cycloaddition to afford 46 and 48, wherein different selectivities and efficiencies were achieved, based on the presence or absence of the cobalt complex. 

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