Method of preparation of esters of new halo - pregnanes, in particular of delta4 - 3, 20 - diketo - 6alpha - fluoro - one is of delta4 - 3, 20 - diketo - 6alpha - methyl - 17alpha, 21 - dihydroxy - pregnenes, of the corresponding compounds having a double bond in position 1, 2 and 21 - acyloxy compounds