Degradations and Rearrangement Reactions
2.7
395
functionalized morpholine 121 is transformed into the pyrrolidine 122 with excellent stereoselectivity. The stereochemistry at the junction of the main product is cis. This protocol has been
applied to the synthesis of inositol phosphate analogs using the 5-C-vinyl glycoside derivative 123 [109] ( > Scheme 29).
⊡ Scheme 29
The O-benzyl derivative of the glycal 126 undergoes stereoselective ring contractions on treatment with thallium(III) nitrate [110] ( > Scheme 30).
⊡ Scheme 30
Triflates of aldonolactones are a productive source of ring-contraction reactions. Compound 128 contracts its ring under acidic and basic conditions to give C-furanosides [111,112]
( > Scheme 31). Triflates of glycosides occasionally yield ring-contracted products [113].
Another paper has provided an additional example of a ring-contraction reaction of a sugar triflate on reaction with tetrabutylammonium nitrite in moist toluene; the triflate yields
a ring-contracted byproduct [114]. It has been found that the ring-contraction reactions of
the triflate in the presence of pyridine depend on the acidity of the solvent; in the acidic
solvent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), ring-contraction of 130 proceeds smoothly.
Oxygenophilic silyl protection of the 2-O-triflate of aldonolactone 131 prohibits ring contraction so that the reaction results in the formation of the silyl-migrated epoxide 132 [115].
Mitsunobu conditions smoothly effect clean ring-contraction reactions of thiosugars [116]
( > Scheme 32). From the thioheptanoid 134, the thiopyranoid 135 is obtained. Mild sulfonation induces a spontaneous ring-contraction of the azaheptanoid 136 to afford the azapyranoside 137 [117].
Some rare, four-membered sugar rings have been synthesized by ring-contraction reactions [103]. Similar to 2-O-triflates, which easily undergo ring-contraction reactions [118].
DAST-treatment of the thiopentofuranose derivative 138 affords the ring-contracted product 139 having a thietane framework ( > Scheme 33). DAST-assisted ring-contraction has
2.7
395
functionalized morpholine 121 is transformed into the pyrrolidine 122 with excellent stereoselectivity. The stereochemistry at the junction of the main product is cis. This protocol has been
applied to the synthesis of inositol phosphate analogs using the 5-C-vinyl glycoside derivative 123 [109] ( > Scheme 29).
⊡ Scheme 29
The O-benzyl derivative of the glycal 126 undergoes stereoselective ring contractions on treatment with thallium(III) nitrate [110] ( > Scheme 30).
⊡ Scheme 30
Triflates of aldonolactones are a productive source of ring-contraction reactions. Compound 128 contracts its ring under acidic and basic conditions to give C-furanosides [111,112]
( > Scheme 31). Triflates of glycosides occasionally yield ring-contracted products [113].
Another paper has provided an additional example of a ring-contraction reaction of a sugar triflate on reaction with tetrabutylammonium nitrite in moist toluene; the triflate yields
a ring-contracted byproduct [114]. It has been found that the ring-contraction reactions of
the triflate in the presence of pyridine depend on the acidity of the solvent; in the acidic
solvent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), ring-contraction of 130 proceeds smoothly.
Oxygenophilic silyl protection of the 2-O-triflate of aldonolactone 131 prohibits ring contraction so that the reaction results in the formation of the silyl-migrated epoxide 132 [115].
Mitsunobu conditions smoothly effect clean ring-contraction reactions of thiosugars [116]
( > Scheme 32). From the thioheptanoid 134, the thiopyranoid 135 is obtained. Mild sulfonation induces a spontaneous ring-contraction of the azaheptanoid 136 to afford the azapyranoside 137 [117].
Some rare, four-membered sugar rings have been synthesized by ring-contraction reactions [103]. Similar to 2-O-triflates, which easily undergo ring-contraction reactions [118].
DAST-treatment of the thiopentofuranose derivative 138 affords the ring-contracted product 139 having a thietane framework ( > Scheme 33). DAST-assisted ring-contraction has