Thiocarbamates rearrangement

FREUDENBERG - SCHdNBERG Xanthate Rearrangement Rearrangement of S-methyl xanthates to S-methyIdithiocarbonatas (conversion of alcohols lo thiols via xanthates, also phenols to thiophenols via thiocarbamates). 

Formation of enantio- and diastereoenriched l-aza-4-oxa-7-thiabicyclo[3.3.0]octan-8-ones 453a and 453b was accomplished by ring closure of acyl-substituted. Y-bcnzyl thiocarbamates 452 in presence of Amberlyst 15 and 1,3-propanedithiol via a rearrangement of the oxazolidine ring (Equation 213) <2000JPR828>. 

Chloroallyl) thiocarbamate sulfoxides (. ., 5-2) un-.doubtedly rearrange in an analogous manner but in tbis case tbe sulfenate quickly undergoes an additional 1,2-elimination reaction (7 ). Tbe resulting products are tbe IJ, -dialkylcarbamoyl-sulfenyl chloride (M) and tbe carbonyl compound, aldehydes... 

While the cyclohexenyl and cycloheptenyl derivatives roc-14aa-14ag and rac-14ba were stable at room temperature, the cyclopentenyl O-allylic thiocarbamates already suffered a partial rearrangement at ambient temperatures [19, 20] while the acyclic O-allylic thiocarbamates showed an intermediate thermal stability. 

Table 2.1.4.12 Pd(0)/BPA-catalyzed rearrangement of racemic acyclic O-allylic thiocarbamates.

The sense and degree of asymmetric induction of the Pd(0)-catalyzed rearrangement of the cyclic and acyclic O-allylic thiocarbamates in the presence of BPA are the same as, or similar to, those in the Pd-catalyzed substitutions of the corresponding cyclic and acyclic racemic allylic carbonates and acetates with sulfinates and thiols. It is therefore proposed that Pd(0)/BPA reacts with the racemic O-allylic thiocarbamate with formation of a jt-allyl-Pd(II) complex, which contains as counter ion the corresponding thiocarbamate ion (Scheme 2.1.4.19) [23, 24]. Substitution of the jt-allyl-Pd(II) complex by the thiocarbamate ion gives the S-allylic thiocarbamate and the Pd catalyst. 

The Pd-catalyzed allylic alkylation of sulfinate ions, thiols, and thiocarboxylate ions with racemic cyclic and acyclic allylic esters in the presence of bisphosphane BPA generally provides for an efficient asymmetric synthesis of allylic sulfones, sulfides, and thioesters. The Pd-catalyzed rearrangements of allylic sulfinates and allylic O-thiocarbamates, both of which proceed very efficiently in the presence of BPA, are attractive alternative ways to the asymmetric synthesis of allylic sulfones and allyUc thioesters also starting from the corresponding racemic alcohols. 

Thioamide formation benzodiazepinone, 505 heteiodiazepinone, 621 phosphorus pentasulf ide, 323, 600 Thioazole formation, nitrile addition, 301 Thiocarbamate formation, 588 phenol, 95 rearrangement, 517 Thioenol ether formation, 185, 517 addition-elimination, 554 Thioester formation, mixed anhydride, 184 Thioether formation, 241, 300, 413, 416 alkylation, 586, 588 aromatic displacement, 416 Thiohydantoin formation, 293 Thiol interchange, benzothiazole formation, 422... 

Trithioisatoic anhydrides were also cyclized to thiazolo-[2,3-bjquinazolines (255) upon reaction with 2-aminoethanol or l-amino-2-dimethoxyethane (80PHA124 83ZC215). Double ring closures occured when jV-(2-methoxycarbonylphenyl)thiocarbamate reacted with 2-aminothioethanol (82JIC1117) to give 256. Thermal molecular rearrange-... 

Similar absolute asymmetric synthesis was demonstrated in the solid-state photoreaction of A-(P,y-unsaturated carbonyl)thiocarbamate 41. [27] Achiral 0-methyl AT-(2.2-dmeth ibut-3-enoyl)-iV-phenylthiocarbarnate 41 crystallized in chiral space group P2i, and irradiation of these crystals gave optically active thiolactone in 10-31% ee. A plausible mechanism for the formation of 42 is rationalized on the basis that photolysis of 41 undergoes [2 + 2] cyclization to thietane and is subsequently followed by rearrangement to thiolactone 42. 

Scheme 33. DoM of aryl O-thiocarbamate. Link to the Newman-Kwart rearrangement and consequent S-thiocarbamate cross-coupling.

Palladium(ll) catalysis is also effective for the rearrangement of alkyl allyl A-aryldithiocar-bimidates to alkyl Ar-allyl A-aryldithiocarbamates65 (see Table 5, entry 5) or 0-alkyl S-allyl iminothiocarbonatcs to 0-alkyl /V-allylthiocarbamates5,64. Only poor diastereoselectivity (3-5% de) was obtained in the palladium(II)-eatalyzed rearrangement of S-allyl-0-[(—)-menthyl]-iminothiocarbonate64 to the thiocarbamate (see Table 5. entry 1). 

The isothiocyanates are transformed into thiocarbamates by reaction with alcohols or into the acetamides by reaction with acetic acid or anhydride and sodium acetate. These transformations have also been applied to the preparation of unsaturated aminosugars73,74. Thus, suprafa-cial rearrangement of ethyl 2.3,4-trideoxy-6-0-methanesulfonyl-4-thiocyan ato-a-D-t/ireo-hex-2-enopyranoside to the 2-isothiocyanatc occurs in refluxing toluene (1 h), whereas the erythro-epimer needs to be heated in dimethylformamide for 6 hours73. This difference in reactivity might be due to an unfavorable anomeric effect and C-6 in an axial orientation in the transition state of the errr/iro-epimer, respectively. 

Combination of a Diels-Alder reaction and a [3,3] sigmatropic rearrangement provides access to 1,4-substituted 2-cyclohexenyl derivatives75- 76,11 . Thus, Diels-Alder reaction of ( )-l,3-bu-tadienyl thiocyanate with methyl acrylate is followed by a [3,3] sigmatropic rearrangement to the isothiocyanate. The isothiocyanate is trapped by the solvent to give racemic 0-cthyl-/V-(4-methoxycarbonyl-2-cyclohexcnyl)thiocarbamate in a 66 33 diastereomeric ratio (cisjtram). A Lewis acid catalyzed variant of this tandem reaction showed better diastereoselectivity (90 10)111. 

The importance of the electronic effect is most evident in pyridine systems bearing nitrogen at the 2 and 4 positions, which activate the rearrangement by protonation, as exemplified by 0-2-pyridyl thiocarbamate (Scheme 8) [6]. The activation is such that the reaction occurs at room temperature. In contrast, where nitrogen is at the 3 position the temperature at which rearrangement occurs is lowered by 60 °C, from 210 to 150 °C in the case of phenyl. 

The position of a trimethylsilyl substituent in phenols 33 determines great yield differences in both thiocarbamate formation and rearrangement [57, 58]. The required TMS-substituted phenols, which were prepared from the respective bromophenols, were tested under standard conditions (Scheme 19). The yield of the rearrangement decreased as the substituent moved from para to ortho. 

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