Abstract
The 1,5-HAT–1,2-(ester)alkyl radical migration (Surzur–Tanner rearrangement) radical/polar sequence triggered by alkoxyl radicals has been studied on a series of C-glycosyl substrates with 3-C-(α,β-d,l-glycopyranosyl)1-propanol and C-(α-d,l-glycopyranosyl)methanol structures prepared from chiral pool d- and l-sugar. The use of acetoxy and diphenoxyphosphatoxy as leaving groups provides an efficient construction of 10-deoxy-1,6-dioxaspiro[4.5]decane and 4-deoxy-6,8-dioxabicyclo[3.2.1]octane frameworks. The alkoxyl radicals were generated by the reaction of the corresponding N-alkoxyphthalimides with group 14 hydrides [n-Bu3SnH(D) and (TMS)3SiH], and in comparative terms, the reaction was also initiated by visible light photocatalysis using the Hantzsch ester/fac-Ir(ppy)3 procedure. Special attention was devoted to the influence of the relative stereochemistry of the centers involved in the radical sequence on the reaction outcome. The addition of BF3•Et2O as a catalyst to the radical sequence resulted in a significant increase in the yields of the desired bicyclic ketals.
Highlights
The development of synthetic methodologies for bicyclic 1,6dioxaspiro[4.5]decane1 and 6,8-dioxabicyclo[3.2.1]octane (6,8-DOBCO)2 scaffolds is largely stimulated by their occurrence as the structural core of highly active insect pheromones.3 They can be widely found as subunits4 in the structure of other more complex and biologically important natural products such as steroids,5 polyether ionophores,6 and marine toxins.7 In some cases, both structural motifs are present in the same natural skeleton, as occurs in pinnatoxins and the related pteriatoxins, potent neurotoxins of a dinoflagellate origin.8 both bicyclic ketals have attracted much interest from synthetic chemists as versatile building blocks in fine organic synthesis.9In the carbohydrate field, the preparation of spiro-heterocycles has been recently reviewed.10 Several naturally occurring 2,7-anhydro-β-D-glyco-hept-2-ulopyranose sugars with 6,8dioxabicyclo[3.2.1]octane structures have been described
We reported on a new procedure for the stereoselective construction of 1,6-dioxaspiro[4.5]decane12 and 6,8-dioxabicyclo[3.2.1]octane13 frameworks on carbohydrate models as described in Scheme 1
Step in the synthesis of cephalosporolide E.36. We have described another example of this sequence during the reaction of methyl 2,3,4-tri-O-acetyl-6-deoxy-α-D-Talp-(1 → 4)-2,3-diO-methyl-α-D-Glcp-6-O-yl disaccharide radical
Summary
The development of synthetic methodologies for bicyclic 1,6dioxaspiro[4.5]decane and 6,8-dioxabicyclo[3.2.1]octane (6,8-DOBCO) scaffolds is largely stimulated by their occurrence as the structural core of highly active insect pheromones. They can be widely found as subunits in the structure of other more complex and biologically important natural products such as steroids, polyether ionophores, and marine toxins. In some cases, both structural motifs are present in the same natural skeleton, as occurs in pinnatoxins and the related pteriatoxins, potent neurotoxins of a dinoflagellate origin. both bicyclic ketals have attracted much interest from synthetic chemists as versatile building blocks in fine organic synthesis.9In the carbohydrate field, the preparation of spiro-heterocycles has been recently reviewed. Several naturally occurring 2,7-anhydro-β-D-glyco-hept-2-ulopyranose sugars with 6,8dioxabicyclo[3.2.1]octane structures have been described. The development of synthetic methodologies for bicyclic 1,6dioxaspiro[4.5]decane and 6,8-dioxabicyclo[3.2.1]octane (6,8-DOBCO) scaffolds is largely stimulated by their occurrence as the structural core of highly active insect pheromones.. The development of synthetic methodologies for bicyclic 1,6dioxaspiro[4.5]decane and 6,8-dioxabicyclo[3.2.1]octane (6,8-DOBCO) scaffolds is largely stimulated by their occurrence as the structural core of highly active insect pheromones.3 They can be widely found as subunits in the structure of other more complex and biologically important natural products such as steroids, polyether ionophores, and marine toxins.. They can be widely found as subunits in the structure of other more complex and biologically important natural products such as steroids, polyether ionophores, and marine toxins.7 In some cases, both structural motifs are present in the same natural skeleton, as occurs in pinnatoxins and the related pteriatoxins, potent neurotoxins of a dinoflagellate origin.. The most representative example is sedoheptulosan (2,7anhydro-β-D-altro-hept-2-ulopyranose), analogous compounds with D-gluco and D-manno stereochemistry are known.
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