Synthesis of 3-alkenyl-2-arylchromones and 2,3-dialkenylchromones via acid-catalysed retro-Michael ring opening of 3-acylchroman-4-ones David S. Clarke, a Christopher D. Gabbutt, a, * John D. Hepworth b and B. Mark Heron a a Department of Colour and Polymer Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK b James Robinson Ltd, PO Box B3, Hillhouse Lane, Huddersfield HD1 6BU, UK Received 27 April 2005; revised 1 June 2005; accepted 10 June 2005 Available online 1 July 2005 Abstract—3-Acylchromones and 3-acylflavones, readily available by acylation of 2 0 -hydroxydibenzoylmethane with acid anhydrides in the presence of base, undergo efficient conjugate reduction with NaBH 4 in pyridine to give the corresponding chroman-4-ones/ flavanones in high yields. The reduction is both regio- and chemoselective. Treatment of the chroman-4-ones with MeSO 3 H gener- ates the 3-alkenyl-2-arylchromones by a dehydrative rearrangement initiated by retro-Michael cleavage of the pyranone ring. This reduction–rearrangement sequence can be extended to 2-alkyl-3-alkenoylchromones to generate 3-alkenyl-2-styrylchromones, the first examples of 2,3-dialkenylchromones. Ó 2005 Elsevier Ltd. All rights reserved. Although 2-styrylchromones are well known and easily accessible from 2-methylchromones 1 other alkenylchro- mones have been much less investigated. 3-Alkenyl- chromones have been obtained from the reaction of spiro-annulated chroman-4-ones with formamide ace- tals, 2a and the acid-catalysed rearrangement of 2-alkyl- and 2,2-dialkyl-3-(hydroxymethylene)chroman-4-ones, 2b a reaction, which we have extended to the synthesis of 3,4-dihydro-9H-xanthen-9-ones 3a and also to 3-isopro- penylthiochromone. 3b Reactions of phosphorus ylides with 3-formylchromone provide 3-alkenylchromones. Thus, acylmethylenetri- phenylphosphoranes afford the corresponding (E) alk- enes, 4a whilst benzylidenetriphenylphosphoranes give, predominantly, the (Z)-3-styrylchromones. 4b However, the reaction with Ph 2 C@PPh 3 is very inefficient, 4c whilst 3-formylchromones react anomalously in a complex ÔMichael–Michael–WittigÕ sequence with (2,4-dioxobut- ylidene)triphenylphosphoranes to give benzophen- ones. 4d (E)-3-(4-Oxo-1-benzopyran-3-yl)acroleins are available via hydrolysis of the cycloadduct of 3-form- ylchromone and H 2 C@CHOEt. 5a Knoevenagel conden- sation of cyanoacetic and malonic acids with 3- formylchromone provides, respectively, chromone 3-acrylonitrile 5b and 3-acrylic acid derivatives, 5c the latter compounds are also accessible from Heck reac- tions of 3-bromochromones. 5d Recently, (E)-3-styrylchr- omones have been obtained by the microwave initiated Knoevenagel reaction of phenylacetic acids with 3-form- ylchromone. 5e Missing from all these routes is an approach which permits access to simple 3-alkenyl-2- arylchromones. Recently, the acid-catalysed condensation of 2 0 -hydroxydibenzoylmethanes with phenylacetalde- hydes has been reported to provide 3-styrylflavones in a one-pot operation. 6 However, this procedure is limited to arylacetaldehydes and is not amenable to the synthe- sis of simpler 3-alkenylflavones. We now report an extension of our earlier work, 2b which provides an entry to 3-alkenylchromones and flavones from the acid-catalysed retro-Michael ring cleavage of novel, readily available 3-aroyl-2-alkylchroman-4-ones. Application of this dehydrative rearrangement to the hitherto unknown 2-alkyl-3-cinnamoylchroman-4-ones is also described. The requisite starting materials, 2 0 - hydroxydibenzoylmethanes 1, are easily available by O-acylation of 2 0 -hydroxyacetophenone followed by Baker–Venkataraman (BV) rearrangement under standard 0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2005.06.058 Keywords: Conjugate reduction; Chroman-4-ones; Chromones; Flavones; Rearrangement. * Corresponding author. Tel.: +44 (0) 113 343 2936; fax: +44 (0) 113 343 2947; e-mail: c.d.gabbutt@leeds.ac.uk Tetrahedron Letters 46 (2005) 5515–5519 Tetrahedron Letters