Urethanes Diels-Alder reactions

Chapter 9, Section 1), azodicarboxylates and related acyl and aroyl azo compounds (37, 2,3-diaza-l-oxabutadiene, Chapter 9, Section 5), acy-lsulfines (38) and -sulfenes (39,4-oxa-l-thiabutadiene, Chapter 8, Section 5), yV-sulfinylurethanes (40), and N-sulfonylamides and -urethanes (41, 2-aza-4-oxa-l-thiabutadiene, Chapter 9, Section 3). Chapters 8 and 9 detail the investigations on each of the systems indicated and should be consulted for citations and full discussions of their 4ir participation in Diels-Alder reactions. 

A-Sulfinylurethanes and 7V-sulfonylamides and -urethanes similarly participate in Diels-Alder reactions with strained and electron-rich olefins [Eq. (42)].104... 

Intramolecular Diels Alder reactions have also been employed. Macdonald and Durst used a urethane linkage to attach the dienophile to a benzocyclobutene in (125), leading ultimately to podophyllotoxin (113) (scheme 46) [110], Another synthesis of podophyllotoxin by Kraus and Wu involves a Diels Alder reaction of a photo-enol to which the dienophile is attached by an acetal linkage (scheme 47) [111]. Unfortunately, the later steps proceed in low yield and involve epimerisation at C-2 to give picropodophyllone (126). 

As a rule, cycloadditions with acyclic -acyl imino dienophiles are useful synthetic reactions which demonstrate excellent regio- and stereo-selectivity. A commonly used source of IV-acyl immonium dienophiles are bis-urethanes such as (13) (equation S), which upon treatment with a Lewis acid and a diene afford Diels-Alder adducts. In this case, (13) reacts with boron trifluoride etherate to afford immonium ion (14) which adds to diene (15), yielding tetrahydfopyridine (16). ... 

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