Chiral (2,5)Pyrido[7<sub>4</sub>]allenoacetylenic Cyclophanes: Synthesis and Characterization
作者:José Lorenzo Alonso-Gómez、Armando Navarro-Vázquez、M. Magdalena Cid
DOI:10.1002/chem.200900316
日期:2009.6.22
Chiral pyridoallenophanes! The synthesis and characterization of chiral acetylenic pyridoallenophanes along with 2,4‐ and 2,5‐pyridyl[Re(CO)4LBr] complexes are reported. The relative configurations of the four stereoisomers have been unambiguously assigned. Chromatographic resolution of the racemate of the C4 stereoisomer was accomplished and mirror‐image CD spectra have been obtained (see figure)
Regioselective Pd-0-catalyzed cross-coupling of substrates, which bear bispropargylic leaving groups with silyl-protected alkynes, has provided access to a variety of 1.3-diethynylailenes, a new family of modules for three-dimensional acetylenic scaffolding. In enantiomerically pure form, these C-rich building blocks could provide access - by oxidative oligomerization - to a fascinating new class of helical oligomers and polymers with all-carbon backbones (Fig. 2). In the first of two routes. a bispropargylic epoxide underwent ring opening during S(N)2'-type cross-coupling. and the resulting alkoxide was silyl-protected, providing 1,3-diethynylailenes (+/-)-8, (+/-)-12 (Scheme 3), and (+/-)-15 (Scheme 5). A more general approach involved bispropargylic carbonates or esters as substrates (Scheme 6-8), and this route was applied to the preparation of a series of 13-diethyrrylallenes to investigate how their overall stability against undesirable [2 + 2] cycloaddition is affected by the nature of the substituents at the allene moiety. The investigation showed that the 1,3-diethynylallene chromophore is stable against [2 + 2] cycloaddition only when protected by steric bulk and when additional relectron delocalization is avoided. The regioselectivity of the cross-coupling to the bispropargylic substrates is entirely controlled by steric factors: attack occurs at the alkyne moiety bearing the smaller substituent (Schemes 9 and 10). Oxidative Hay coupling of the terminally mono-deprotected 1.3-diethynylallene (+/-)-49 afforded the first dimer 50, probably as a mixture of two diastereoisomers (Scheme 12). Attempts to prepare a silyl-protected tetraethynylallene by the new methodology failed (Scheme 13). Control experiments (Schemes 14 - 16) showed that the Pd-0-catalyzed cross-coupling to butadiyne moieties in the synthesis of this still-elusive chromophore requires forcing conditions under which rapid [2 + 2] cycloaddition of the initial product cannot be avoided.