Ylide rotamers

The reaction of 196 with phenyl chlorocarbene 198 illustrates the synthesis of indolizines by cyclization of pyridinium ylides (Scheme 7). Cyclization of ylide rotamer 199 generates the intermediate product 200, which undergoes elimination of chloride to provide compound 201 <2005EJ01532>. 

The synthesis of 47 and 48 is more difficult than the synthesis of the nonfunctionalized system 37b, in part because the lack of symmetry in the triptycene leads to a mixture of three interconvertible rotamers that must be isolated and identified. Nevertheless, the synthetic strategy developed is heavily based on the previous synthesis (see Sect. 6) of 37b. As summarized retrosynthetically in Scheme 10, the key steps for the synthesis of 47 and 48 are the preparation and photocyclization of the stilbene 54 containing the triptycene unit. Stilbene 54 would be formed by a Wittig reaction between aldehyde 55 and the ylide 56. Photocyclization and cleavage of the methyl ether would give phenol 53 which should allow the incorporation of tethers of variable length to give 47 or 48. 

The paper which reported the photo-induced ring opening of 3-pyridylcar-bene (17) (see Section 3.1) also described a study of the matrix photolysis of 3-azidopyridine (45) (Scheme 9). Irradiation of (45) at 222 nm in Ar at 7 K gave a product with IR and UV absorptions which indicated that it was the ring-opened ylide (47) two rotamers were identified. The ylide also underwent a 1,7-H shift to give the ketenimine (48) in two rotameric forms. 

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