Not even in older literature? Then how to interpret the statement by WittenThere is no such "result".
Thanks! Can you give a reference for that?I'm not up to date to all the stringy constructions of backgrounds, but I think the problem you're addressing is deeper: it has to do with supersymmetry. E.g., pure N=1 D=4 supergravity can be constructed in a anti-deSitter background, but not a deSitter background. The reason is that the Jacobi identities of the underlying algebra don't allow for one particular sign of the cosmological constant (corresponding to dS).