Tapas K. Mandal

Facile Condensation of Aromatic Aldehydes with Chroman-4-ones and 1-Thiochroman-4-ones Catalysed by Amberlyst-15 under Microwave Irradiation Condition

Different aromatic aldehydes and cinnamaldehyde undergo cross-aldol condensation with chroman-4-ones and1-thiochroman-4-ones in the presence of amberlyst-15 under microwave irradiation in solvent free condition to afford rapidly the corresponding E-3-arylidene and E-3-cinnamylidene derivatives, respectively, in high yield. This process is simple, efficient and environmentally benign.

An Expeditious and Safe Synthesis of Some Exocyclic α,β-Unsaturated Ketones by Microwave-Assisted Condensation of Cyclic Ketones with Aromatic Aldehydes over Anhydrous Potassium Carbonate

A rapid, efficient, and solvent-free methodology for synthesis of exocyclic α,β-unsaturated ketones of the categories E-3-arylidene-4-chromanones, E-2-arylidene-1-tetralones, E-2-arylidene-1-indanones, E-3-cinnamylidene-4-chromanones, E-2-cinnamylidene-1-tetralones, E-2-cinnamylidene-1-indanones, α,α′-(E,E)-bis(arylidene)-cycloalkanones, and α,α′-(E,E)-bis(cinnamylidene)-cycloalkanones has been developed through cross-aldol condensation of the constituent cyclic ketones and aldehydes by microwave irradiation over anhydrous potassium carbonate. However, for condensation of 1-thio-4-chromanones with aromatic aldehydes by this method, the initially formed exocyclic α,β-unsaturated ketone has been found to undergo isomerization yielding 3-(arylmethyl)thiochromones.

trans-2-Phenyl-4-thiophenoxy-3,4-dihydro-2H-1-benzothiopyran

Iodine-catalyzed cyclocondensation of cinnamaldehyde and thiophenol yields rapidly trans-2-phenyl-4-thiophenoxy-3,4-dihydro-2H-1-benzothiopyran in excellent yield with very high diastereoselectivity.

Simple Diastereoselective Synthesis of (2S*,2′R*,3′R*,4S*)-2,3′-Diphenylspiro[chroman-3,2′-oxiran]-4-ols and (2S*,2′S*,3′S*,4S*)-2,3′-Diphenylspiro[chroman-3,2′-oxiran]-4-ols

Oxiranes (epoxides) are very versatile building blocks in organic synthesis, and they can be opened up with a wide range of nucleophiles. Nucleophilic addition to epoxides plays a pivotal role in the stereoselective preparation of 1,2-difunctional compounds. It has been one of the most thoroughly studied reactions of epoxides, and for this different promoters and conditions have been developed. 1,3-Disubstituted and 1,2,3-trisubstituted 2,3-epoxyalcohols belong to an important group of epoxy compounds, as opening of their epoxy ring can lead to interesting compounds having three contiguous stereogenic centers. This fascinating aspect is the key factor for their use as precursors for the synthesis of a wide range of important and useful organic molecules. Two main routes for synthesis of 2,3-epoxyalcohols are i) epoxidation of allyl alcohols, and ii) chemoselective ketone reduction of a,b-epoxyketones. Both allyl alcohols and a,b-epoxyketones are readily available compounds, and those belonging to the latter type are generally prepared by a very simple reaction, viz., epoxidation of a,b-unsaturated ketones with hydrogen peroxide and alkali. It is worthwhile to mention here that for the reduction of a,b-epoxyketones, along with the chemoselectivity, the stereoselectivity of the reactions plays a very important role in the transformation steps necessary for achieving the targeted synthesis. This has led to the development of a good number of methods for chemoselective and stereoselective reduction of a,b-epoxyketones. E-3-Benzylideneflavanones (1) prefer a conformation in which the 2-aryl group is axially oriented. Sodium borohydride reduction of 1, studied in our laboratory, was

Schmidt Reaction of E-3-Benzylidenechromanones and E-3-Benzylidenethiochromanones

On treatment with NaN3/c. H2SO4-HOAc or NaN3/TFA, E-3-benzylidenechromanones are mostly converted to E-β-aminobenzylidenechromanones while E-3-benzylidenethiochromanones are converted to 3-benzoylthiochromones. A structurally new type of product has been isolated for the reaction of E-3-benzylidene-4′-methoxychromanone with NaN3/TFA. Mechanistic paths have been suggested for formation of the products.

NBS Oxidation of E-3-Benzylidenechromanones to 3-(α-Hydroxybenzyl)chromones and 3-Benzoylchromones

Both natural and synthetic chromones show diverse biological activities,1–7 some being in regular clinical use.8,9 Thus, the chemistry of chromones and related compounds are of interest to organic chemists with the goal of generating new biologically active molecules. We previously reported that E-3-benzylideneflavanones (4) are readily converted to 3-(α-hydroxybenzyl)flavones (5) by oxidation with NBS in CaCl2-dried CCl4 under reflux conditions.10,11 The products were probably formed through the intermediate 3-(α-bromobenzyl)flavones which underwent rapid hydrolysis with adventitious water. However, attempted isolation of such intermediate did not meet with success. In order to assess the scope of this reaction, we undertook the study of the reaction of NBS with E-3benzylidenechromanones (1) in ordinary CCl4 (commercial AR grade, which usually contains trace amount of moisture), particularly because there are very limited number of published reports of synthesis of the expected products 3-(α-hydroxybenzyl)chromones (2).12,13 Thus, when E-3-benzylidenechromanones 1a-g (1 equiv.) were refluxed in CCl4 with NBS (1 equiv.) and benzoyl peroxide (trace), 3-(α-hydroxybenzyl)chromones (2) were formed in very good yields along with 3-benzoylchromones (3) as minor products (Scheme 1, Table 1). Since the formation of 3 indicated that a further oxidation had occurred, this led us to study the reaction of 1 with double the amount of NBS which led to 3 as the major product and 2 the minor one (Table 1). When the 3-(α-hydroxybenzyl)chromones (2) were treated separately with NBS (1 equiv.), they were completely converted to 3benzoylchromones (3) (Table 2). All the new compounds of the series 2 and 3 gave satisfactory combustion analyses and spectral data. This study was extended to E-3-benzylideneflavanones (4) with two molar equivalents of NBS; Scheme 2, Table 3 shows that, except for 4d, all the substrates gave 3benzoylflavones (6), albeit in low yields. Oxidation of 5a-c [NBS (1 equiv.), (PhCOO)2