Chiral Diels–Alder reaction between cyclopentadiene and nitroso derivatives: thermal isomerisation/racemisation of the adducts

Cyclopentadiene nitroso adducts 1a and ent-1a were synthesised in good yields and good enantiomeric excess from chiral chloro-nitroso derivatives 4 and 5 in the d-mannose and d-ribose series, respectively. The thermal racemisation of these adducts occurred below room temperature. Some other chiral Diels–Alder nitroso adducts were prepared in the d-mandelic, l-prolinol and d-O-methylprolinol series and their thermal isomerisation was specified according to the N-substitution. A simple synthesis of the chiral amido-cyclopentenol (+)-2b, an important precursor for biological interesting compounds, is presented from ent-1a.

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(1R,4S,αR)-3-(α-Hydroxyphenylacetyl)-2-oxa-3-aza-bicyclo[2,2,1]hept-5-eneC13H14NO3de = 90%[α]D25=-78 (c 0.25, CHCl3)Source of chirality: the precursor and d-mandelic acidAbsolute configuration: (1R,4S,αR)

(1S,4R,αR)-3-(α-Hydroxyphenylacetyl)-2-oxa-3-aza-bicyclo[2,2,1]hept-5-eneC13H14NO3de = 96%[α]D25=+69 (c 0.25, CHCl3)Source of chirality: the precursor and d-mandelic acidAbsolute configuration: (1S,4R,αR)

(1S,4R)-4-tert-Butylcarbonylamino-cyclopent-2-en-1-olee = 89%[α]D=+67[α]D=+67 (c 1, CHCl3)Source of chirality: the precursorAbsolute configuration: (1S,4R)