Chroman mechanism

The reaction of 2-polyfluoroalkylchromones (e.g., 323) with l,3,3-dimethyl-3,4-dihydroisoquinolines (e.g., 324) gave zwitterionic 6,7-dihydrobenzo[ ]quinolizinium compounds such as 326 (Scheme 70). The mechanism proposed for this transformation involves an addition-elimination displacement of the chromane heterocyclic oxygen by the enamine tautomer of the dihydroisoquinoline, followed by intramolecular cyclization of the intermediate 325 <20030L3123>. 

The infrared spectra and dielectric constants,101,102 electronic,103-111 nuclear magnetic resonance (NMR),106,112-121 and mass122-126 spectral data for these compounds have been analyzed. There exist numerous quantum mechanical calculations of the electronic transitions and involvement of the sulfur electrons.107,111 Ultraviolet spectral determinations of the basicity of thiochroman-4-ones110 allowed correlations with the corresponding chroman-4-ones. 

Reaction of chlorotrimethylsilane with hydroxymethylspiroepoxycyclohexadienone (279) affords the spiroannelated chroman (280) (Scheme 73) (80TL4973). The proposed mechanism invokes a quinone methide intermediate which dimerizes to the chroman. 

The synthesis of chroman 15 involves an acid-catalyzed condensation of trimethyl hydroquinone (14) with methyl vinyl ketone (9), but unfortunately the mechanism is not clarified in detail and the role of the ortho formate is also not clear. Scott et al. assumed that the reaction proceeds via the intermediates 27 and 28. ... 

Trost, B.M. et ah. Synthesis of chiral chromans by the Pd-catalyzed asymmetric allylic alkylation (AAA) scope, mechanism, and applications, J. Am. Chem. Soc., 126, 11966, 2004. 

As for the mechanism by which this quinone is formed, it should in the first place be said that it can by no means be the same as the one of K2( o), e.g., from phylloquinone. In other words, it is to be excluded that the side chain is introduced as a unit after complete removal of the original side chain of the precursor. This result follows from the finding that all the compounds examined were labeled with tritium in the position l, 2 of the side chain or, in the case of the tocopherol, in position 3,4 of the chroman... 

Scheme 7.56 Oxa-Michael/Michael cascade reaction for the synthesis of chromanes and the mechanism proposed.

This is an explanation for the fact that this type of vulcanization is not enabled by double bonds per se, without aUyic hydrogens in the elastomer molecules [4,74]. (It should be pointed out that the phenolic curative can also act by a slightly different mechanism to give crosslinks which contain chromane structural moieties [75], the allylic hydrogens stUl being required.)... 

To circumvent this structural limitation, an intermolecular process would be desirable [58]. Bromoglycal 201 and easily available propargylic halide 202 react in a nucleophilic substitution. With this product in hand, the Pd-catalyzed domino process is launched and a second alkyne 203 needed to access the ring is taken out of the reaction mixture. Chromans of type 200 were obtained (Scheme 6.36). In a similar manner, isochromans 204 were assembled. The mechanism is similar to the one depicted in Scheme 6.35. To obtain good yields, an excess of the second alkyne... 

On one hand, the chroman head is capable of donation of a hydrogen atom to free radicals leading to the stabilization of these species. On the other hand, the long lipophilic phytyl tail provides high compatibility and miscibility with UHMWPE. The stabilization mechanism of a-tocopherol in UHMWPE have been studied in depth [20, 86]. The cycle of oxidative stabilization of UHMWPE by means of this antioxidant can be found in Ref [87]. 

The second type of mechanism, associated with Greth, suggests the formation of chroman structure between the benzene ring of the re-... 

---