2-silyloxy-l,3-butadiene

Thioxanthenes are not only valuable synthetic intermediates [1], they also find considerable interest in pharmaceutical research [2] and as dyes for numerous applications [3]. Only few methods are known for the stereoselective synthesis of S-heterocycles [4], Sequential transformations are a powerful strategy in organic synthesis frequently allowing the synthesis of complex molecules in a single synthetic operation [5]. It was anticipated that 4-silyloxy-l-benzothiopyrylium-salts 1 [6], which represent double-activated 4-thiochromanones, can be used for the stereoselective synthesis of aimulated thiochromanones 3 by reaction with 2-silyloxy-l,3-butadienes 2 [7]. 

The all-c/s-stereochemistry of the annulation product 9a was unambiguously confirmed by X-ray crystal structure analysis [9]. Subsequently, the reactions of the 4-silyloxy-l-benzothiopyrylium-salt la with a number of 4-substituted-2-silyloxy-l,3-butadienes and 3,4-disubstituted-2-silyloxy-l,3-butadienes (Table 1) were studied. In each case only one out of 4 diastereomers was isolated, namely the all-cis-stereoisomer. Yields are ranging from 68 to 91 %. 

The keto-silylenolether 17 was obtained by 2,6-lutidiniumtrifIate mediated reaction of 6 with 8a. After 50 % consumption of the 2-silyloxy-l,3-butadiene 8a the reaction was worked up and besides 50 % of the starting material 8a 19 % of 9a and 16 % of 17 were isolated. 

Summary A new and efficient method for the highly diastereoselective annulation of 4-quinolones using 4-silyloxyquinolinium-salts is presented. Reactions of the quinolinium-salts with several 2-silyloxy-l,3-butadienes give cis-cis fused acridones with yields of 62-99 %. 

Amuilations of pyridinium- and quinolinium-salts however have not been reported so far. From the addition reactions of C-nucIeophiles like Grignard, organozinc and organotin reagents to pyridinium- and quinolinium-salts have been reported to be valuable methods for the construction of substituted dihydropyridine [4a-4d] and dihydroquinoline-derivatives [4d-4e] results of our studies in the field of benzothiopyrylium-salts [5] we concluded that the sequential intermolecular 1,2-addition/intramolecular 1,4-addition of 4-silyloxyquinolinium-salts 1 with 2-silyloxy-l,3-butadienes 2 might be useful for the diastereoselective preparation of annulated quinolones 3. 

As expected, the reaction of the 4-silyloxyquinolinium-salt 1, which can be regarded as a double activated 4-quinolone, with the unsubstituted 2-silyloxy-l,3-butadiene 2 diastereoselectively gives the cis-flised acridone 5 with 86 % yield. 

A great advantage of the new method is that both reactants, the double activated 4-silyloxyquinolinium-salt 1 and the 2-silyloxy-l,3-butadiene 2 [6], are obtained in situ from the corresponding 4-quinolone 6 and methyl vinyl ketone 7 by reaction with triisopropylsilyltrifluoro-methanesulfonate (TIPSOTf) and TIPSOTfy2,6-lutidine, respectively. 

With trisubstituted benzoquinones and use of the cationic oxazaborolidinium catalyst B, 2-[tra-(isopropyl)silyloxy]-l,3-butadiene reacts at the monosubstituted quinone double bond. The reactions exhibit high regioselectivity and more than 95% e.e. With 2-mono- and 2,3-disubstimted quinones, reaction occurs at the unsubstituted double bond. The regiochemistry is directed by coordination to the catalyst at the more basic carbonyl oxygen. 

DIELS-ALDF.R REACTIONS Bis(methyl-thio)-1,3-ketadiene. 1,3-Bis(trimethyl-silyloxy)-l,3-butadiene. 2,5-Bis(tri-melhylsilyloxy)furanes. 5,5-Dimethoxy-1,2,3,4-tetrachlorocyclopenladiene. Di-peroxo-oxohexamethylphosphoramido-molybdenum(VI). I.thynyl p-tolyl sulfotie. I nrane. (E)-l-Metlloxy-1.3-... 

Summary Sequential intermolecular 1,2-addition/intramolecular 1,4-addition-reactions of 4-silyloxy-l-benzothiopyrylium-salts with 2-silyloxy-1,3-butadienes highly diastereo-selectively give annulated thiochromanones. 

It is remarkable that both reactants, the 4-silyloxy-l-benzothiopyrylium-salts 1 as well as the 2-silyloxy-1,3-butadienes 2, are formed in situ in one pot. First, the 4-silyloxy-l-benzothiopyrylium-salt 1 is obtained from the thiochromen-4-one 6 by treatment with a trialkylsilyltriflate [8] and then, the 2-silyloxy-1,3-butadiene 2 is formed by treatment of methyl vinyl ketone 7 with a trialkylsilyltriflate and 2,6-lutidine in the same flask. 

DIELS-ALDER REACTIONS Benzocyclopropene. 1,2-BisC3-tosyIethoxycarbonyl)-diazene. Cyclobutadiene iron tricarbonyl. 1,2-Dicyanocyclobutene. Diethyl ketomalonate. 1,3,4,6-Heptatetraene. 2-Hydroxy-5-oxo-5,6-dihydro-2/f-pyrone. 3-Hydroxy-2-pyrone. Isopropylidene isopropylidenemalonate. Lithium tetra-methylpiperidide. 4-Methoxy-5-acetoxymethyl-o-benzoquinone. 4-Methoxy-5-methyl-o-benzoquinone. frans-l-Methoxy-3-trimethyl-silyloxy-l,3-butadiene. Per-fluorotetramethylcyclopentadiene. 4-Phenyl-l, 2,4-triazoline-3,5-dione. Potassium f-butoxide. 

Related Silanes. l-Trimethylsilyl-l-(ethoxyethoxy)allene (5) undergoes efficient reaction with electrophiles to produce substituted unsaturated acylsilanes (eq 5). The reaction products can be converted to diverse 2-silyloxy-1,3-butadienes. Exposure of (5) to boron trifluoride etherate leads to rearrangement in which an oxonium ion is intercepted at C-2 of the allene (eq 5). This rearrangement proceeds via an intermolecular pathway. Acylstan-nanes can also be prepared through this method. 

The enantioselective intramolecular formal 2+4-cycloaddition of acrylates and a, -unsaturated imines (99) catalysed by chiral phosphines (100), derived from amino acids, produced A-heterocycles (101) (Scheme 31). Chiral dirhodium(II) carboxamidates (102) catalysed the hetero-Diels-Alder reactions between 2-aza-3-silyloxy-l,3-butadienes and aldehydes to yield all cw-substituted l,3-oxazinan-4-ones in high yields and high enantioselectivity (98% ee)P The nickel-catalysed 4 + 2-cycloaddition of a, -unsaturated oximes with alkynes yielded 2,3,4,6-tetrasubstituted pyridine derivatives. The reaction of isoquinoline, an activated alkyne, and 4-oxo-4//-l-benzopyran-3-carboxaldehyde (103), in ionic solvents, produced 9a//,15//-benzo[a][l]benzopyrano[2,3-/t]quinolizine derivatives (105) via the zwitterion (104) selectively and in good yields (Scheme 32).The Diels-Alder cycloaddition of ethyl 3-(tetrazol-5-yl)-l,2-diaza-l,3-butadiene-l-carboxylates with -rich heterocycles, nucleophilic olefins, and cumulenes formed 3-tetrazolyl-l,4,5,6-tetrahydropyridazines regioselectively. The silver-catalysed formal inverse-electron-demand Diels-Alder... 

The sequential reaction of the 4-silyloxy-l-benzothiopyrylium-salt 1 with 4-phenyl-2-silyIoxy-1,3-butadiene 8a exclusively yields the all-cu-annulated thiochromanone 9a with high yield. 

Table 1. Diastereoselective sequential transfonnations of 4>silyloxy>l-benzothiopyrylium-salt la wifli 4-substituted and 3,4-disubstituted 2-silyIoxy-1,3 butadiene ...

HEXAHYDRO - 4a,5 - DIMETHYL - 2(3H) - NAPHTHALE-NONE and 2-TRIMETHYLSILYLOXY-1,3-BUTADIENE AS A REACTIVE DIENE DIETHYL trans -4-TRIMETHYL-SILYLOXY-4-CYCLOHEXENE-l,2-DICARBOXYLATE. Sulfur substitution also continues to be of high interest, and three preparations on sulfide synthesis are included BENZYL SULFIDE DIALKYL AND ALKYL ARYL SULFIDES NEOPENTYL PHENYL SULFIDE and UNSYMMETRICAL DIALKYL DISULFIDES sec-BUTYL ISOPROPYL DISULFIDE. 

Related Reagents. 1-Acetoxy-1,3-butadiene 2-Methoxy-1,3-butadiene 2-Methoxy-3-phenylthio-1,3-butadiene 1-Methoxy-3-trimethylsilyloxy-l,3-butadiene 1 2-Trimethyl-silyloxy-1,3-butadiene. 

The study of dienophilic character of monosubstituted nitro-aromatic pentaheterocycles was carried out employing 2-nitrofuran la 2,5-dimethyl-3-nitrofuran lb, l-tosyl-2-nitropyrrole 2a, l-tosyl-3-nitropyrrole 2b, 2- and 3- nitrothiophene 3a, 3b, and 2- and 3-nitroselenophene 4a, 4b. In addition, isoprene 5, l-N-acetyl-N-propyl-l,3-butadiene 6, 1-methoxy-3-trimethylsiloxy-l,3-butadiene (Danishefsky s diene) 7 and l-diethyl-amino-3-ferf-butyldimethyl-silyloxy-1,3-butadiene (Rawal s diene) 8 were selected as dienes, covering an attractive sp>ectre of nucleophilic character (Figure 1). 

Preparation by oxidative cycUzation of l,3-bis(trimethyl-silyloxy)-l-methyl-l,3-butadiene to a acetonitrile solution of sodium bicarbonate (12 equiv.) and CAN (6 equiv.) was slowly added an acetonitrile solution of l,3-bis(trimethyl-silyloxy)- -methyl-1,3-butadiene (2 equiv.) at -45°. 

TroponesReaction of vinyldiazomcthanes (1) with l-methoxy-l-(trimethyl-silyloxy)butadiene (2) catalyzed by rhodium(H) acetate or rhodium(ll) pivalate results in [3+4]cycloaddition via cyclopropanation/Cope rearrangement to form a cyclohcptadiene (3). Short exposure of 3 to citric acid followed by oxidation (DDQ) provides the... 

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