Dithioacetals, a-oxoketene

The Junjapjpa-Ila (JI)-Heteroaromatic Annulation A New General a-Oxoketene Dithioacetals Mediated Inverse Method for the Synthesis of Benzo/Condensed Heterocycles and Related Heteroaromatization Processes... 

Table 1. List of a-Oxoketene Dithioacetals 6 Used in the Present Work ...

The a-oxoketene dithioacetals 6 (6.1-6.43) employed in this work as three carbon 1,3-biselectrophilic components have been drawn from various active methylene ketones and are described in the Table-1. The corresponding a-oxoketene N,S- and 0,S-acetals (17.1- 17.15) which are usually derived from a-oxoketene dithioacetals are described in Table-2. These examples are only a representative groups to demonstrate the general application of the new Jl-heteroaromatic annulation methodology... 

Reaction of 3-MethyI-5-lithiomethyIisoxazole 18 with a-Oxoketene dithioacetals 6 A New General Method for the Synthesis of Substituted and Fused 1,2-Benzisoxazoles (Type I, la Model)... 

Synthesis of Benzoheterocycles from Heterocyclic Ketone Derived a-Oxoketene Dithioacetal Precursors... 

The above examples represent Jl-heteroaromatic annulation involving the reaction of allyl anions whose double bond is a part of the heterocyclic ring system (Scheme 1). The corresponding a-oxoketene dithioacetals (1,3-electrophilic component) were generally derived from nonheterocyclic carbonyl precursors. Alternatively the Jl-heteroaromatic annulation can also be employed to a-oxoketene dithioacetals derived from heterocyclic ketones (1,3-bielectrophile) and hetero/nonheteroallyl anions (1,3-binucleophile). These reactions are described below. 

The a-oxoketene dithioacetal 6.40 is derived from indoxyl (l,2-dihydroindol-3-one), a heterocyclic carbonyl precursor, and its reaction with simple allyl anions will also yield the corresponding Jl-annulation product. Thus when 6.40 was reacted with allyl anions 65 the corresponding carbinol acetals 66 formed insitu underwent smooth BF3.Et20 assisted cyclization to afford the corresponding carbazoles 67 in high yields <99T11563>. 

The reaction of a-oxoketene dithioacetals with dimethylsulfonium methylide led to another general route to furans and butenolides [454], according to the accompanying scheme. 

Methallylmagnesium chloride (l).5 The reaction of 1 with the a-oxoketene dithioacetal 2 provides a 1,2-adduct 3 that on treatment with HBF4 is converted into the aryl sulfide 4. 

Use of a-oxoketene dithioacetals and organocuprates provided a stereoselective synthesis of tri- and tetra-substituted alkenes.109 In fact, it was reported that either the (E)- or (Z)-alkene could be prepared by the proper choice of cuprate and of the sequence of addition (Scheme 21). 

A synthesis of a-pyrones has been repotted where a key step involved the chemoselective addition of an organocuprate to an a-oxoketene dithioacetal.115 The methodology requires four steps and is quite general since the cuprate can deliver a range of ligands in a selective manner (Scheme 22). 

Oxaziridines, 276, 277 2-Oxazolidones, chiral, 379-381 2-Oxazoline-5-ones, 151 Oxazolines, 147, 265, 588 Oxime sulfonates, 245-246 jx-Oxobis(chlorotriphenylbismuth), 381-382 Oxodiperoxymolybdenum(pyridine)(hexamethyl phosphoric triamide), 382-383 p,3-Oxohexakis( x-triinethylacetato)trimethanol-triiron(ffl), 382-383 a-Oxoketene dithioacetals, 185 Oxomethoxymolybdenum(V) 5,10,15,20-totraphcnylpoiphyrin, 383 Oxone, 442 Oxosulfenylation, 205... 

The a-carbon atom of a-oxoketene dithioacetals (51, n = 1, 2) reacts with aromatic aldehydes to give 1 1 and 2 1 adducts, i.e. Baylis-Hillman and double-Baylis-Hillman , using mediation by titanium tetrachloride.180... 

Ketene thioacetals 7 have been used to synthesize thiophene with a carbonyl in the 3-position 8 (Scheme 2) <03X1557, 03X2631>. In the reported procedure <03X2631> a-oxoketene dithioacetal is converted to the substituted thiophene 9 upon reaction with diiodomethane in the presence of Zn-Cu. Simple formylation or acetylation of thiophenes only produces the 2- or 5-derivatives and, hence, this is a useful technique to produce the 3- or 4- derivatives. Xhis method also has been studied to synthesize substituted selenophenes 10 <03SL855>. 

Oxoalkane nitriles, use for heterocyclic synthesis 84S1. a-Oxoketene dithioacetals as three-carbon synthons 86T3029. 

Reduction of a-oxoketene dithioacetals. The double bond is saturated to give dithioacetals of (i-keto aldehydes. Note that different sets of products are obtained on reduction by the Zn-ZnCl ATMEDA system. ... 

---