5  4,281,117  6  (w ).  s  (CH3)2N-CH=Nt(>  ;  N  Cl  0  .  COOR’  .•.  W  -+-f)s  .H-C-NH  L:  ;  N  Cl  0  COOR  I  peracid  ~  oxidation  0  II  W  -+-f)s  H-C-NH  L:  .  ;  N  Cl  0  COOR  t  reduction  u-LNnt( ✓ s  ;  N  Cl  .  0  COOR  The strong acid salts are prepared by treating a solu› tion of the free base form of the compound of the above  formula in an organic solvent with the strong acid or a  The salts of the above formula formed with strong  acids are stable salts useful for storage of quantities of  the compound for subsequent use in the preparation of  the 3cchloro nucleus by the process of this invention. As  noted above the salts formed with strong acids are sta› ble to chemical degradation of the: 7-amidino side chain.  Accordingly, these salts are converted to the free base  form prior to use in the second step of the process.  The formation of the sulfoxide can be carried out  with a peracid such as peracetic, perbenzoic or prefera› bly m-chloroperbenzoic acid in a suitable solvent such  as acetone or methylene chloride. The 3-chloro-3- cephem ester sulfoxide is readily isolated by evapora- 65  tion of the reaction mixture and dissolution of the reac› tion product mixture in an organic solvent such as ethyl  acetate. The m-chlorobenzoic acid side product is  The carboxy-protecting ester groups,  R’,  are the same  carboxy-protecting groups defined for  R  above and  include preferably p-nitrobenzyl, p-methoxybenzyl or  diphenylmethyl.  The compound of the above formula wherein  R’  is  hydrogen is obtained by the de-esterification of a car› boxy-protected ester derivative to either the salt form  or free base form of the compound. The free acid com› pound can be used as the intermediate in the process  which is preferably re-esterified with another ester moi› ety for conversion to the N-formyl product.  wherein A is phenyl, tolyl, chlorophenyl, or naphthyl.  wherein  R’  is hydrogen or a carboxy-protecting group;  n is 0 or 1, and salts thereof formed with a strong acid  selected from the group consisting of hydrochloric  acid, hydrobromic acid, sulfuric acid, phosphoric acid,  or a sulfonic acid represented by the formula  In a further aspect of the invention there are provided  the intermediates useful in the above process and repre› sented by the following formula  As mentioned above, when a carboxylic acid or a  phenol is used in the process the N-formyl 3-chloro  product of the process is obtained as a mixture of the  isomeric 2-cephem and 3-cephem and, under basic re- covery conditions, as the 2-cephem isomer. The mixture  of isomers or the 2-cephem isomer can be converted to  the desired N-formyl 3-chloro-3-cephem ester by the  known isomerization method which involves forming  the sulfoxide of the 3-chloro ester followed by reduc- tion back to the sulfide form. The formation of the  sulfoxide causes the 2,3-double bond to isomerize to the  3,4-position (3-cephem). The isomerization is illustrated  by the following reaction scheme.  The 7,8-formamido-3-chloro-3-cephem-4-carboxylic  acid ester is converted to the desired 7-amino-3-chloro- 3-cephem ester by acid hydrolysis of the N-formyl  group. The hydrolysis is carried out in a solvent such as  methyl alcohol, tetrahydrofuran, or mixtures thereof  • with hydrochloric acid. For example, the N-formyl-3- chloro ester is dissolved in a mixture of methyl alcohol  and tetrahydrofuran and the solution is treated at room  temperature with a small amount of concentrated hy› drochloric acid. The 7-amino-3-chloro nucleus ester is  obtained as the hydrochloride salt.  The reduction of the N-formyl 3-chloro-3-cephem  ester can be carried out by known sulfoxide reduction  methods. For example, the reduction methods de› scribed by Murphy et al., U.S. Pat. No. 3,641,014, and  by Hatfield, U.S. Pat. No. 4,044,002. The latter method  is an especially useful reduction method in that the  recovery of the desired product is readily achieved.  washed from the solution with base and, after the solu› tion of the product is dried and evaporated, the product  can be further purified if desired by chromatography.  forming groups commonly used to block or protect the  C4 carboxylic acid group of the cephalosporins. Exam› ples of such groups include benzyl and substituted ben› zyl such as p-methoxybenzyl, p-nitrobenzyl, and 2,4,6- trimethylbenzyl; diarylmethyl groups such as diphenyl- methyl, 4-methoxydiphenylmethyl, and 4,4’ -dimethox› ydiphenylmethyl; and alkyl and substituted alkyl groups  such as t-butyl, t-amyl, 2-iodoethyl, 2-bromoethyl, and  2,2,2-trichloroethyl. Such ester forming groups are  functional groups in that they serve for the temporary  protection of the carboxy group. After such use they  are removed by known hydrolytic or reduction meth› ods to provide the free carboxylic acid.  The 3-hydroxy-3-cephem ester employed as the start› ing material in the process is prepared as described in  U.S. Pat. No. 3,917,587, issued Nov. 4, 1975. 