Chroman ring

The carbanions of 1-alkenyl sulphoxides 400 also react with carbonyl compounds to give the corresponding condensation products384 (equation 237). Solladie and Moine have used this type of reaction in their enantiospecific synthesis of the chroman ring of a-tocopherol 401. Addition of the lithio reagent 402 to the aldehyde 403 affords the allylic alcohol 404 in 75% yield as a sole diastereoisomer481 (equation 238). 

In this chapter, we have described some useful properties of coordinated QM complexes. For instance, remarkably stable T 2-o-QM complexes of Os can release the QM moiety upon oxidation, followed by controlled reaction with suitable dienophiles to form the chroman ring system.15 16 ri2-Coordinatedp-QM complexes can be easily released upon exposure to an electron-deficient alkene and trapped as the 1,6-Michael addition product.18 19... 

The mechanistic routes to (15e) have been investigated. Labelling experiments revealed that the expelled methyl radical originates separately from C-2, C-3 and C-4 of the chroman ring by three competing pathways <70OMS(3)753). A similar observation has been made for... 

The chroman ring system is stable to organometallic reagents, for example in the formation of the tertiary alcohol (690) in high yield (63HCA650), and to the usual interconversion of carboxylic acid, acyl chloride, carboxamide and nitrile. 

The levels of cardiovascular factors could also be influenced by the ability of vitamin E to affect activation of arachidonic acid from membrane phospholipids by phospholipase A2. Vitamin E, either given in the diet or by incubation with platelets themselves, was found to inhibit phospholipase A2 in a dose-dependent manner. a-Tocopheryl acetate had little or no effect on the activity of this enzyme, but tocol, without methyl groups in the chroman ring, was more potent than either (+)- or ( )-a-tocopherol, suggesting that the methyl groups were not important for the inhibition but the hydroxy group in the ring was critical for activity [131]. 

This effect was seen only in hypoxia, not normoxia, was dose-dependent within the range 0.2-200 /rM, and provided a simple in vitro model for investigating the mode of action of the vitamin. By examining the structure-activity relationship of the response, compounds with a phytyl side-chain, phytol and vitamin K-l (phytomenadione), of similar length to vitamin E (9) were found to be also active, but compounds that had structures which resembled the chroman ring of the vitamin, vitamin K-3 (menaphthone) and Trolox, were antagonists of the responses to the phytol side-chain effects. 

The cyclisation of o-alkenylphenols features in two approaches to the chroman ring system. The Hg-mediated cyclisation of (7) affords the chroman-4-ols (8) and (9) which can be separated after debenzylation, providing the chroman unit of the calophyllum coumarins (95S630). 2-Cyclohexenylphenols undergo a 6-endo cyclisation to fused chromans on treatment with pyridine hydrobromide perbromide (95CJC1727). 

Reviews are available on natural products that contain the chroman ring system <2000NPR193, 2001NPR310>, and the synthesis of compounds that modulate potassium channels, many of which feature a chroman moiety <1996S307>. 

Thermolysis of a benzodioxan leads to a cycloadduct processing the chroman ring system, a key intermediate during synthetic studies on communesin B <20030L3169>. 

Rotenoids have a basic structural element involving fused chromone and chromane rings. The best-known rotenoid is the furanorotenoid rotenone (C501 Phe pyran-... 

The key features of a new total synthesis of vitamin E from 2,3,5-trimethylhydroquinone are an enantioselective Wacker cyclisation which generates the chroman ring system with the... 

Deshpande, P. R Baker, D. C. A simple approach to the synthesis of the chiral substituted chroman ring of calophyllum coumarins. Synthesis, 1995, 630-632. 

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