Retro-hetero-ene reactions

Deprotonation of one of the Me groups adjacent to S gives an ylide which can undergo a retro-hetero-ene reaction to give the observed products. 

The conversion of 5 to 6 is a Swern oxidation. The O of DMSO is nucleophilic, and it reacts with oxalyl chloride. Cl- then comes back and displaces O from S to give a S electrophile. The OH of 5 is then deprotonated, whereupon it attacks S, displacing CL. Then deprotonation of a Me group and a retro-hetero-ene reaction occur to give the ketone. 

Although this last step is reasonable, it is not the way that the elimination actually takes place. In the currently accepted mechanism, the CH3 group is deprotonated to give a sulfur ylide, and a retro-hetero-ene reaction ensues. Retro-hetero-ene reactions are discussed in Chapter 4. 

The elimination of an acid such as AcOH or PhSeOH may indicate a retro-ene or retro-hetero-ene reaction. 

If (CD3)2SO (deuterated DMSO) is used for the Swem reaction, the E2 mechanism predicts that the sulfide product should be (CD3)2S the retro-hetero-ene mechanism predicts that it should be (CD3)S(CHD2). Guess which product is actually found ... 

As a rule, the initial hetero-DiELS-ALDER adduct 2 cannot be isolated. It eliminates N2 in a retro-DiELS-Alder reaction and is converted into a 4,5-dihydropyridazine 3. This can be stabilized as a 1,4-dihydropyridazine 7 (especially if X = H) by a 1,5 hydrogen shift or (if X = OR and NR2) as the pyridazines 5 and 6 by dehydrogenation or HX elimination. As a diazadiene, it can also engage in a further Diels-Alder reaction with excess of alkene 3delding the stable 2,3-diazabicyclo[2.2.2]oct-2-ene 4. The initial Diels-Alder product tetraazabicyclo[2.2.2]octatriene 8, which arises from the reaction between alkynes and 1,2,4,5-tetrazines, undergoes a cycloreversion with N2 elimination affording the pyridazine 6. With nitriles, 1,2,4-triazines 9 are obtained. 

Vogel and co-workers have used 1 as double chain elongation synthon in their synthesis of long chain polypropionate fragments (eq 48). Their approach is based on an one-pot reaction cascade which involves hetero-Diels-Alder addition of electron-rich l-alkoxy-3-acyloxy-l,2-dienes (e.g., 25) to sulfur dioxide, subsequent ionization into zwitterionic intermediates (e.g., 26), then reaction of the latter with allylsilanes. The final step of this sequence is the retro-ene desulfinylation of an intermediate allylsilyl sulfinate (e.g., 27) to produce useful synthetic intermediates containing two more stereogenic centers (e.g., 28). 

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