Merck and Co. Inc.
Patent: US2951082 , 1956 ;

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Hoffmann-La Roche
Patent: DE1044103 , 1957 ;

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Am. Cyanamid Co.
Patent: GB834143 , 1956 ;

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Bonvicino et al.
Journal of Organic Chemistry, 1961 , vol. 26, p. 2383,2387