Thionitroso compounds Diels-Alder reactions

Thiophene 1-oxide is unusually reactive both as a diene and as a dienophile in the Diels-Alder reaction and thiophene S,N-ylids apparently exhibit similar reactivity. When electron-rich alkenes, (e.g., acenaphthalene) are treated with 60 (R = C02Et) (Scheme 10), a rapid reaction ensues to yield the product derived from a [4 -f- 2]-cycloaddition followed by cheleotropic elimination of a thionitroso compound. This reaction appears to be fairly general for nonhindered alkenes, and even relatively unreactive systems, such as thiophene itself, give low yields of 61. This is an unusual reaction... 

One example of what is perhaps an inverse electron demand Diels-Alder reaction of a thionitroso compound has appeared [Eq. (29)].56... 

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