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Synthesis of alkenyl sulfides and ketenedithioacetals from thiolesters and dithiocarbonates
R1 R2 R3 Time (min) Yield (%) (Z)/(?) ratio
Ph Me Me 30 77 80:20
Ph Ph Me 420 56 82:18
Ph Me n-Bu 60 75 84:16
Ph Me PhCHj 80 80 81:19
n-C8H17 Me Me 20 94 73:27
n-QHiv Ph Me 300 71 68:32
n-C8H,7 Me PhCH2 30 87 90:10
n-C8H17 Me c-C6H„ 30 95 82:18
c-QH„ Me Me 20 88 94:6
c-C6HM Ph Me 180 79 91:9
c-CeHn Me PhCHj 30 95 100:0
c-C6HM Me c-C6H„ 20 97 100:0
S-(CH2)3 Me 20 72
S-(CH2)3 n-Bu 45 76
S-(CH2)3 PhCH2 30 87
s-(cm c-QH„ 75 86
concentrated and the residue was purified by column chromatography on silica gel using hexane/ethyl acetate. Ketene dithioacetals were prepared as mentioned above and purified by distillation under reduced pressure after addition of K2C03 and filtration of the mixture with Hyflo Super-Cel.
From  with permission.
We have mentioned previously (see Section 3.1) the easy formation of a double bond by sulfoxide pyrolysis. Associated with an alkylative step of a metallated sulfoxide, the sequence is of great value in alkene synthesis [44U, 441], as in the one-pot reaction given here.
4.3.4 Two-fold extrusion
The olefin bond can be formed by a “two-fold extrusion” [442-444].
VY — R')=<RS
^,/^Y^V -X. -Y R»/ R?
Ä3-1,3,4-Thiadiazolines were easily prepared from ketones or thioketones. By pyrolysis in the presence of a phosphine they afforded olefins [128,441] (see Section 4.5.4).
RaC CRs N=N
Extrusion of carbon dioxide and sulfur was also used by Barton , in the following example:
These methods were mainly used in the synthesis of crowded tetrasubstituted alkenes (in search of tetra(t-butyl)ethylene).
4.3.5 The Eschenmoser reaction
The utility of intermediaiy sulfur bridging has been evidenced by the Eschenmoser group . An example of their method was applied in studies  directed towards the synthesis of vitamin B,2.
The preparation of an enamine ester from thiopyrrolidone is a key step in a total synthesis  of (±)-isoretronecanol (2).
Phosphorus pentasulfide (24.4 g, 110 mmol) and anhydrous xylene (300 ml) were mixed in a 1000 ml flame-dried three-necked flask equipped with an efficient mechanical stirrer. Pyrrolidone (42.5 g, 500 mmol) was added in one portion and the solution was allowed to stir at room temperature for 20 min. A dark-brown oil separated after the reaction mixture was heated at 130°C for 30 min with vigorous stirring. The hot solution was filtered through a coarse sintered glass funnel. White crystals formed immediately in the filtrate. Fresh xylene (100 ml) was added to the oily residue, and the solution heated to 130°C for 20 min followed by a hot filtration. This process was repeated once more and the combined xylene was allowed to cool. White crystals were collected by filtration, and air dried to give thiopyrrolidone (32.2g, 64%), m.p.
From  with permission.
Alkylation of thiopyrrolidone with ethyl bromoacetate; preparation of an enamine ester
Ethyl bromoacetate (53.3 g, 319 mmol) was added dropwise to thiopyrrolidone (32.2 g, 319 mmol) dissolved in dichloromethane (250 ml) at 0°C. The reaction mixture was allowed to warm gradually to room temperature and was stirred for 4h. Solid NaHCO, was added in portions until further addition did not cause COz evolution. The dichloromethanc solution was washed with saturated NaHC03 and water, dried over MgS04 and concentrated to give the intermediate thioimidoester (60.0 g, 100%), b.p. 115-120°C/1.2 torr.
The thioimidoester (60.0 g, 319 mmol), triphenylphosphine (334 g, 1.28 mol) and anhydrous xylene (550 ml) were combined in a flame-dried 1000 ml three-necked flask. A solution of potassium t-butoxide (3.57 g, 32.0 mmol) in t-butyl alcohol (20 ml) was added dropwise and the reaction mixture was stirred at room temperature for 4h and then refluxed for 60 h. It was cooled and extracted three times with 10% HCI (3X 150 ml). The combined aqueous extracts were washed three times with ether (3 X 50 ml), neutralized with solid sodium hydrogencarbonate and extracted with four portions of dichloromethane (4 X 100 ml). The combined dichloromethane layers were washed twice with brine (2 X 80 ml), dried over MgS04 and concentrated to give a crude brown solid which was sublimed (40°C/0.1 torr) to yield (1) (37.6 g, 76%), m.p. 62-63°C.