2.3- dihydro-, conformation 2 + 3 cycloaddition

IH-Azepine, 1-methoxy carbonyl-cycloaddition reactions, 7, 522 with nitrosobenzene, 7, 520 tricarbonyliron complex acylation, 7, 512-513 conformation, 7, 494 tricarbonylruthenium complex cycloaddition reactions, 7, 520 1 H-Azepine, l-methoxycarbonyl-6,7-dihydro-synthesis, 7, 507... 

The BF3-catalyzed reaction of a-aminoaldehydes with 10 is valuable for highly stereoselective synthesis of 2,3,5-trisubstituted pyrrolidines with all -cis configurations (Equation (50)).197 The stereochemical outcome like chelation-controlled stereochemistry might result from the inherent conformational arrangement of the aldehyde-BF3 complex. />-Quinoneimines, o-quinones, and a-alkoxyhydroperoxides undergo similar types of [3 + 2]-cycloadditions with allylsilanes to afford dihydro indoles, dihydrobenzofurans, and 1,2-dioxolanes, respectively.164,175,198... 

For A-acyliminium ions that (can) adopt the s-cis conformation, the mechanistic picture is quite different. Now the /V-acyliminium intermediate reacts as a 4ir-electron component in a Diels-Alder cycloaddition with inverse electron demand (equation 28). " This process also shows high regio- and stereo-selectivity in most cases. A nice illustration of high stereospecificity is found in recent work on the intramolecular Diels-Alder reaction of A-acyliminium species (equations 29 and 30). The bis-amides (50) and (51) serve as precursors to the reactive intermediates, which cycloadd to the alkenes with high selectivity to give r/a s-fused bicyclic 5,6-dihydro-1,3-oxazines. 

Similarly, the planar chiral tricarbonylchromium complexes of ortho substituted benzaldimines are useful for a variety of stereoselective cycloaddition reactions. For example, the benzaldimine chromium complexes gave aza-Diels-Alder product, 2,3-dihydro-4-pyridinone chromium complexes with high diastereoselectivity by reaction with Danishefsky s diene (Eq. 18) [11,17,18]. The high diastereoselectivity of the cycloadducts is also based on the preferred antz-conformation of the starting benzaldimine chromium complexes as well as the planar chiral benzaldehyde chromium complexes. The cycloaddition of imines having arene chromium complex at the remote position with Danishefsky s diene underwent smoothly in good yields, but the diastereoselectivity was low (Eq. 19) [17]. 

A four-carbon tether (49a) yields only the trans-isomer. This is likely due to a high level of strain in the polycyclic product 50a. The crystal structure of a dihydro derivative of 50a found the tether-derived cyclohexane to be in a boat conformation, a substantial contribution to the strain of this adduct. An analog carrying an additional methyl group at a carbon involved in bond formation, 49b, does not undergo cycloaddition. ... 

---