Glycosyl Imidates, 66. Synthesis of the Tetrasaccharide Moiety of Plant Growth Regulator Calonyctin A

3-O-Benzyl-protected quinovose 6 was transformed into 1,2-O-unprotected derivative 9 which on treatment with TBS-Cl in the presence of a base gave selectively 2-O-unprotected glycosyl acceptor 10. Similarly, 3-O-allyl-protected quinovose 11 was transformed into 1,2-O-unprotected derivative 14. 1,2-O-Acetylation of 14, selective removal of the 1-O-acetyl group with hydrazinium acetate, and subsequent treatment with trichloroacetonitrile in the presence of DBU furnished the versatile 2-O-acetyl-3-O-allyl-protected quinovosyl donor 17. Reaction of donor 17 with acceptor 10 in the presence of TMSOTf as the catalyst gave disaccharide 19. Treatment of 19 with NaOMe/MeOH provided 2b-O-unprotected derivative 20 which gave with rhamnosyl donor 18 in the presence of TMSOTf as the catalyst trisaccharide 21. 3b-O-Deallylation of 21 and subsequent reaction with donor 17, again in the presence of TMSOTf as the catalyst, gave target tetrasaccharide 2. Removal of all O-protective groups furnished D-Quis(13)[L-Rhaα(12)]D-Quis(12)D-Qui (3), the tetrasaccharide moiety of calonyctin A.