Chemoselective strategies strategy

Scheme 1 Examples of Chemoselective Ligation Strategies Resulting in the Formation of Nonpeptidic Bonds (a) Hydrazone, (b) Oxime, (c) Thiazolidine...

R. Geurtsen, F. Cote, M. G. Hahn, and G.-J. Boons, Chemoselective glycosylation strategy for the convergent assembly of phytoalexin-elicitor active oligosaccharides and their photoreactive derivatives, /. Org. Chem., 64 (1999) 7828-7835. 

Scheme 9 HWE/chemoselective reduction strategy toward mono(benzannulated) spiroacetals [53]...

The selectivity for two-alkyne annulation can be increased by involving an intramolecular tethering of the carbene complex to both alkynes. This was accomplished by the synthesis of aryl-diynecarbene complexes 115 and 116 from the triynylcarbene complexes 113 and 114, respectively, and Danishefsky s diene in a Diels-Alder reaction [70a]. The diene adds chemoselectively to the triple bond next to the electrophilic carbene carbon. The thermally induced two-alkyne annulation of the complexes 115 and 116 was performed in benzene and yielded the steroid ring systems 117 and 118 (Scheme 51). This tandem Diels-Alder/two-alkyne annulation, which could also be applied in a one-pot procedure, offers new strategies for steroid synthesis in the class O—>ABCD. 

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