Macrocycles and linear unsaturated compounds are not the only molecules constructed using the power of metathesis. Oligosaccharides benefit from metathesis too! One of the goals in proteomics research is to understand the functional impact of post-translational modification, and protein glycosylation represents the most complex post-translational event. Over 50% of human proteins are glycosylated; however, the role of carbohydrates in glycoproteins is relatively unknown. Blechert and coworkers saw the possibilities metathesis had for exploring oligosaccharides. They used selective cross coupling of monosaccharide building blocks to explore carbohydrate arrays. The key is a selective yne-ene cross metathesis between a terminal alkyne and a terminal alkene to yield 1,3-disubstituted butadienes, followed by a subsequent MeAlCl2-catalyzed Diels– Alder transformation with methyl vinyl ketone (e.g., X = CH; Acc = C(O)Me).
Like other cross metathesis reactions, the formation of E/Z mixtures results in diastereomers relative to the cyclohexene configuration; however, a simultaneous cleavage of the acetates and equilibration to the more stable trans-configuration occurs using NaOMe in MeOH–THF. This strategy was also applied to the synthesis of pipecolic acid derivatives using an aza-Diels–Alder (e.g., X = N; Acc = Ts). This technique allows the preparation of carbohydrate-containing molecules with great structural and functional diversity, which means that metathesis is indeed sweet!