Isothiouronium salts esters

Some strategies used for the preparation of support-bound thiols are listed in Table 8.1. Oxidative thiolation of lithiated polystyrene has been used to prepare polymeric thiophenol (Entry 1, Table 8.1). Polystyrene functionalized with 2-mercaptoethyl groups has been prepared by radical addition of thioacetic acid to cross-linked vinyl-polystyrene followed by hydrolysis of the intermediate thiol ester (Entry 2, Table 8.1). A more controllable introduction of thiol groups, suitable also for the selective transformation of support-bound substrates, is based on nucleophilic substitution with thiourea or potassium thioacetate. The resulting isothiouronium salts and thiol acetates can be saponified, preferably under reductive conditions, to yield thiols (Table 8.1). Thiol acetates have been saponified on insoluble supports with mercaptoethanol [1], propylamine [2], lithium aluminum hydride [3], sodium or lithium borohydride, alcoholates, or hydrochloric acid (Table 8.1). 

A. Drop 1 g of sodium into 10 ml of methanol in a small flask provided with a small water condenser heat the mixture until all the sodium has dissolved. Cool, and add 1 g of the ester and 0.5 ml of water. Frequently the sodium salt of the acid will be deposited either at once or after boiling for a few minutes. If this occurs, filter off the solid at once, wash it with a little methanol and convert it into the p-bromophenacyl ester, p-nitrobenzyl ester or S-benzyl isothiouronium salt (for experimental details, see Section 9.6.15, p. 1261). If no solid separates, continue the boiling for 30-60 minutes, boil off the alcohol, allow to cool, render the product just neutral to phenolphthalein with dilute sulphuric or hydrochloric acid, convert the sodium salt present in solution into a crystalline derivative (Section 9.6.15, p. 1261) and determine its melting point. 

Selenonothiolcarbamic acid esters from isothiouronium salts... 

Replacement of imino groups by oxygen Thiolcarbamic acid esters from isothiouronium salts... 

The mechanism of the reaction of the vinyl carbinol (2) with cyclic 1,3-diketones has been discussed [1077]. New 1,3-dicarbonyl compounds have been condensed with this carbinol. Thus, the reaction with 2-chloro-l,3-cyclopentanedione led to an analog of 18-norestrone (1) with an aromatic ring C [1078]. The condensation of the isothiouronium salt of the vinyl carbinol (2) (see Scheme 38) with 4-acetoxy-2-methyl-l,3-cyclopentane-dione gave an unstable highly unsaturated compound (3) the catalytic hydrogenation and demethylation of which led to 8-isoestrone with an overall yield of 28% on the (2) [1079]. The reaction of the vinyl carbinol (2) with a, y-dimethyltetronic acid led to compound (4). Treatment with pal-ladized carbon and alkaline hydrolysis gave the keto ester (5) and the electrochemical reduction of this yielded racemic bisdehydrodoisynolic acid (6) [1080]. 

---