1 -Thio-1,2-isopropylidene acetals

Mootoo and co-workers disclosed a procedure for the preparation of C-l substituted galactals based on the intramolecular capture of an oxocarbenium ion by an enol ether residue.79 In their approach, the key intermediate 1 -thio-1,2-O-isopropylidene acetals (TIA) are activated with methyl triflate to generate the crucial annulating synthon (1,2-O-isopropylidenated oxocarbenium ion). 

Thio-l, 2-0-Isopropylidene Acetals Annulating Synthons for Highly Hydroxylated... 

Analogues of complex saccharides, in which one or other of the acetal oxygens is replaced by a methylene residue or other heteroatoms, have received attention as biochemical probes and as potential therapeutic agents. The use of l-thio-1,2-isopropylidene acetals (TIA s) as annulating synthons for highly hydroxylated systems is illustrated by the synthesis of (3-C-, p-aza-C- and P-carba- galacto disaccharides. 

The synthesis of 4 and 5 provided an opportunity to evaluate the scope of a new C-glycosidation methodology that was used for 3 (28-34). Accordingly, esterification (step A) of the glycone component, l-thio-l,2-0-isopropylidene acetal (TIA) 16 and one or the other aglycone segments, C-branched saccharide acids 14 or 15, furnishes ester 12 or 13, respectively (Scheme 1). Tebbe methylenation (step B) of the latter provides enol ethers 10 or 11. Thiol... 

Thio-D-ribose (20) was first reported by Clayton and Hughes [44,45], an improved s)mthesis from D-ribono-1,4-lactone was published in 2002 [46]. Isopropylidene acetals [47] as well as 1,4-anhydro derivatives thereof [48] have also been made available. 

Quite recently, a new access to KDO from 1-thio-l, 2-O-isopropylidene acetal 196 has been reported [122]. Its condensation with 2-furoic acid, followed by the Tebbe methylenation afforded 198 (Scheme 42). Cyclization process was mediated by methyl triflate. Thus obtained 1-furano glycal derivative 199 was converted into KDO derivative 66 and its 2-deoxy-a-analog 200 in a reaction sequence shown in Scheme 43 [122]. 

Khan N, Cheng X, Mootoo DR (1999) l-Thio-l,2-0-isopropylidene acetals novel precursors for the synthesis of complex C-glycosides. J Am Chem Soc 121 4918-4919... 

The acetolysis of S-acetyl-l,2-0-isopropylidene-3,5-di-0-methyl-6-thio-a-D-glucofiiranose with acetic acid—acetic anhydride—sulfuric acid gave a small yield of the crystalline septanose triacetate (271). The septanose structure of 271 was established by the absence of the thiol band in its infrared spectrum, and by the signals for three O-acetyl and two O-methyl groups found in its nuclear magnetic resonance spectrum. 

As early as 1961/62 the first examples of 5-thiopyranoses were reported in the literature. Adley and Owen [4] described the synthesis of 5-thio-L-idopyranose (3) by thiirane ring opening of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene- 8-L-idofuranose with potassium acetate in a mixture of glacial acetic acid and acetic anhydride and subsequent deprotection. 

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