147976-35-0Relevant articles and documents
Regioselective Lewis acid-directed reactions of 2-alkoxy-5-alkyl-1,4-benzoquinones with styrenes: Synthesis of burchellin and guianin neolignans
Engler,Wei,Letavic,Combrink,Reddy
, p. 6588 - 6599 (2007/10/02)
Reactions of 2-carbomethoxy-5-methoxy-1,4-benzoquinone with (E)-propenylbenzenes promoted by Ti(IV) regio- and stereoselectively yield trans 2-aryl-4-carbomethoxy-7-methoxy-3-methyl-2,3-dihydro-5-benzofuranols. However, the products found in Lewis acid-promoted reactions of 2-alkoxy-5-alkyl-1,4-benzoquinones with (E)-propenylbenzenes depend on the nature of the Lewis acid. Low-temperature reactions with SnCl4 produce rel-(1R,6S,7S,8R)-4-alkoxy-1-alkyl-7-aryl-8-methylbicyclo[4.2.0]oct-3- ene-2,5-diones 21-23 and products derived from them. Allowing the reactions to warm to room temperature results in rel-(2S,3S,3aR)-3a-alkyl-2-aryl-5-hydroxy-3-methyl-2,3,3a,6-tetrahydro -6-oxobenzofurans as mixtures of keto-enol tautomers 26/27 and 28/29. With Ti(IV) as promoter, the reactions can be made to produce either 21-23 or the regioisomeric rel-(1S,6R,7R,8S)-4-alkoxy-1-alkyl-8-aryl-7-methylbicyclo[4.2.0]oct-3- ene-2,5-diones 18-20. The ratio of the two depends upon the makeup of the Ti(IV) promoter and the substituents on the propenylbenzene. A mechanistic rationale is presented involving regioselective coordination of the different Lewis acids to the quinone. Thus, SnCl4 binds to the C-1 carbonyl and C-2 alkoxy oxygens, forming a complex possessing a 2-alkoxy-5-alkyl-4-oxo-2,5-cyclohexadienyl carbocation moiety (e.g. 30) which undergoes a thermally allowed 4π + 2π (5 + 2) cycloaddition with propenylbenzene to give a bicyclo[3.2.1]octenyl carbocation 31; this cation then rearranges to the observed products. On the other hand, the Ti(IV)-promoted reactions give either bicyclo[3.2.1]octenyl carbocation 31 or 34, depending upon reaction conditions, followed by rearrangement. Cyclobutanes 18-20 undergo protic acid rearrangement to give trans 7-alkoxy-4-alkyl-2-aryl-3-methyl-2,3-dihydro-5-benzofuranols 15-17, whereas cyclobutanes 21-23 yield 5-alkyl-7-aryl-3-hydroxy-6-methylbicyclo[3.2.1]oct-3-ene-2,8-diones 24/25. The reactions provide an efficient and stereoselective route to neolignans of the guianin and burchellin classes.
STEREOSELECTIVE SYNTHESES OF THREE DIFFERENT CLASSES OF NEOLIGNANS FROM THE SAME STARTING MATERIALS
Engler, Thomas A.,Wei, Dong,Letavic, Michael A.
, p. 1429 - 1432 (2007/10/02)
Lewis Acid catalyzed reactions of 2-alkoxy-5-allyl-1,4-benzoquinones with styrenes can be manipulated to form either the burchellin or guianin neolignans, or 4-allyl-2-aryl-3-methyl-2,3-dihydrobenzofurans.