6838 Table I.
Preparation of Thexylalkyl(2-ethoxycarbonyletheny1)boranesand Their Conversion into fl-Hydroxycarboxylic Acids
--__
Alkyl group (R) of t hexylmonoalkylborane
Hydroboration product-Yield (2b 3b) by p m , " Isomer distribution," %
Cyclohexyl Norbornyl 3-Methyl-2-butyl trans-2-Meth ylcyclopent yl 2-Methyl-1-pentyl
+
%
2b
3b
94 93
>99
Trace
90 >99 >99 67
Trace Trace 33
101 89 94
7 -
/3-Hydroxycarboxylic acidsb--
%---
-Yield, Glpcc
Isolated
60
74 57 72 81 46d
10
51 68 69
MP, "C 79.5-80.0 80-82 50.5-51.5 69.5-70.0
e
Based on pmr examination of vinyl protons (benzene as a n internal standard). The chemical-shift ranges: H, (d, J = 19 Hz) 7.63-7.70; All new, isolated products yielded satisfactory H b (d, J = 19 Hz) 5.90-6.04; H, (d, J = 3 Hz) 6.40-6.42, and Hd (d, J = 3 Hz) 5.31-5.40. By glpc examination after treatment of the product with analytical and spectral data. c By glpc examination of silylated products (SE-30). an excess of diazomethane (SE-30). e Isolated by glpc as methyl 2-methoxy-4methyloctanoate. a
dicating that steric effect can be significant in determining the orientation in this reaction. Accordingly, we tested a series of thexylm~noalkylboranes~ containing a highly bulky thexyl group as hydroborating agents. Fortunately, all but one of the thexylmonoalkylboranes of different steric requirements we tested placed at least 90% of the boron atoms in the desired p positions, producing only minor amounts of the isomeric products 3b6 (Table I). Moreover, the usual reluctance of the thexyl group to participate in the ionic reactions of organoboranes10 was observed in the subsequent reaction of 2b with base, thereby making the onepot procedure shown by eq 2 a highly practical syn-
I 1 " R2
I I R"C-CCOIR' I I
I I I I H SCH,
R'-C-CCO,R'
H
R1 R2
I I
I 1
H
S
R'--C-CCO-R' ..
R1\ +
,R!
C,=C, R4
C0.R'
0K! 'CH,
I exemplify the process.8 No attempt has been made to optimize the yields listed. The thermolysis of the a-methylsulfinyl derivative of ethyl decanoate was studied in some detail. At temperatures below 120", elimination was sluggish. At 120" 5 hr was required. In refluxing toluene, completion occurred in about 14 hr. No difference in yield was noted for the two methods. Following the reaction by nmr revealed the exclusive formation of the E isomer during the entire course of the elimination. Entry 5 illustrates the utility of a lactone as substrate. The same a-methylthiolactone (1) is also available by the methylation of a-methylthio-y-butyrola~tone.~This
Department of Chemistry, Emory Unicersity Atlanta, Georgia 30322
0
(4) LhN
Receiced h f u y 31, 1973
CH 1
0
C H , S d 0 CH 1
New Synthetic Reactions. Sulfenylation-Dehydrosulfenylation as a Method for Introduction of Unsaturation
Sir: Introduction of unsaturation a,p to a carbonyl group serves as a major method for elaboration of organic structures. Invariably, the basic approach involves bromination followed by dehydrobromination. The wide diversity of brominating agents and "bases" attests to the capriciousness of the method. Application of this method to esters and lactones' becomes even more difficult because of the lack of general methods to effect direct a halogenation.2 Because of our needs to carry out such a transformation under very mild conditions, we developed a new method based upon the facile direct sulfenylating of enolates. 3,3g (1) (a) A.E. Greene, J. C. Miller, and G. Ourisson, TetrahedronLett., 2489, 3375 (1972); (b) T. R. Kastur, G. R. Pettit, and I
