Aldol cyclization thermal

Furthermore, as described by Mori and coworkers, the domino aldol/cyclization reaction of the 3-keto sulfoxide 2-422 with succindialdehyde (2-423) in the presence of piperidine at r.t. afforded the chromone 2-424 which, on heating to 140 °C, underwent a thermal syn-elimination of methanesulfenic acid to provide 2-426 in 22 % overall yield (Scheme 2.100) [227]. This approach was then used for the synthesis of the natural products coniochaetones A (2-425) and B (2-427) [228]. 

A further example of the use of 2//-thiopyrans as surrogates for m-substituted dienes involves the use of the protected 3,4-dihydio-3-(3-oxobutyl)A//-thiopyranA-onc, 3-[2-(2-methyl-l,3-dioxolan-2-yl)ethyl]-4-[tris(l-methy-lethyl)silyl)oxy-2//-thiopyran 328 as an equivalent of l-ethenyl-2-methylcyclohexene in Diels-Alder reactions. The thiopyran reacted with various maleimides to yield the endo cycloadducts and with methyl propenoate to give the exo adduct under either thermal or Lewis-acid-catalyzed conditions. In the latter case concomitant release of the protected ketone functions occurs, acid-catalyzed cyclization of which generates a fused cyclohexenone ring (Scheme 67). Desulfurization, preferably before the aldol cyclization, leads to derivatives of 2,3,4,4a,5,6,7,8-octahy-dro-4a-methylnaphthalenes < 1997CJC681 >. 

Lange and his co-workers have reported a short, convenient approach to the sesquiterpene (—)-acorenone (170) centred about the stereoselective Michael addition of l-methoxybut-3-en-2-one to the less hindered face of the enamine (169) followed by aldol cyclization, ° and Oppolzer etal. have synthesized acorenone and several other members of the same sesquiterpene family by further elaboration of the cis,trans mixture of esters (171) formed, with 100% cndo-selectivity, during the intramolecular thermal ene reaction of the 1,6-diene (172). ... 

Phenols add intramolecularly to Michael acceptors. " Under acidic conditions, a one-pot sequence starts with initial electrophilic acetylation of the activated aromatic ring and is followed by cyclization." With an appropriate leaving group in the /f-position (OMe. or other amines such as in the unsaturated carbonyl compound (e.g., 4) is formed. Other approaches to pyroncs include the self-condensation of protected //-hydroxy acrylates,intramolecular aldol reactions followed by condensation,thermal cycli-zations of unsaturated ()-chloro esters,and an iodo-cyclization-elimination sequence w th Michael acceptors.Oxymercuration of an unsaturated alcohol is an alternative cyclization approach to tetrahydropyrans. 

Compound 24 forms when 23 reacts with 2-aminobenzaldehyde 17 in a normal aldol condensation where the methyl group is converted to a nucleophile. In the second step, the amino nitrogen of 17 attacks the imine carbon of 23 with subsequent loss of N-NH-Ph. Alternatively, 23 was condensed with o-nitrobenzaldehyde and reduction and thermal cyclization gave pyrazoloquinoline 25 (34J1C427), a procedure similar to that used in the synthesis of an indolinquinoline (60CIL1871). 

Furan is possibly a carcinogenic substance. It occurs in heated food, especially in roasted coffee. Isotopic dilution analyses with [ H4]-furan as the internal standard yielded 2.5-4.3 mg/kg furan in differently produced coffee powders. Baby food, e.g., carrot mash and potato/spinach mash contained 74 and 75 pg/kg respectively. Furan is formed from amino acids which yield acetaldehyde and glycolaldehyde on thermal degradation (Fig. 9.4). Aldol condensation, cyclization and elimination of water are the reaction steps. Other precursors of furan are carbohydrates, polyunsaturated fatty acids and carotinoids (Fig. 9.4). Furan can also be formed from the thermolysis of ascorbic acid. 

If one starts with the separately prepared enolsilylether 81, mercury-catalyzed cyclization gives a high yield of 79, while the easy-to-make aldol 82 leads to 80 after a thermal retro-aldol process (34). 

Use as a Base. TBAF has been widely used for a variety of base-catalyzed reactions such as alkylation, elimination, halogenation, Michael addition, " aldol condensation, and intramolecular cyclizations. - It is especially useful when other inorganic bases face solubility problems in organic solvents. The reactions are usually carried out below 100 °C due to the low thermal stability of TBAF. ... 

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