SYNTHESIS OF MEDIUM RING AZACYCLES VIA ALLENE-BASED CYCLIZATIONS - EVALUATION OF POSSIBLE MECHANISTIC PATHWAYS

Two pathways have been evaluated to account for the I-2-mediated cycli sation of the allenic sulfonamides 1a-e based on (i) the steric demand s of the allenic unit and (ii) the ambident reactivity of the sulfonam ide anion. The 1,3-disubstituted allenic sulfonamides 6 and 7 have bee n prepared and both undergo a facile cyclisation. without the need of an external base,;to give the smaller of the two possible ring sizes 8 and 11, respectively. In,the case of 7, the intermediate allylic iodi des 9 and 10 have been observed and steric factors appear to play:a de cisive role in determining the preference for ring size in this cyclis ation sequence. A possible role for-the sulfonamide unit acting as an ambident nucleophile by undergoing initial O-alkylation has also been examined. The sulfonimidates 15a-c undergo thermal [3,3]-rearrangement to give the corresponding sulfonamides 16a-c but the conditions requi red to achieve this O- to N-migration step would preclude involvement of this pathway in the I-2-mediated cyclisation of allenic sulfonamide s.