Hydrogen iodide, ether cleavage with

Gaseous or aqueous hydrogen iodide is the common reagent for cleavage however, it also leads to extensive reduction of the product. A modification which overcomes this difficulty consists in heating the ether with otthophosphoric acid and potassium iodide, viz., ... 

Zinnes and co-workers prepared pyrido[l,2-b][l,2]benzothiazines (Scheme 8) 1,2-benzothiazine 182 with isopropyl iodide and potassium carbonate resulted in spontaneous aldol cyclization of the intermediate enol ether 183 to 7,8-dihydro-8-hydroxy-ll-isopropyloxy 8-substituted pyrido[l,2-b][l,2]benzothiazin-10(9//)-one 5,5 dioxide (184). In sulfuric acid, dehydration and ether cleavage of 184 gave the corresponding unsaturated )3-diketone 185. Hydrogenation gave the saturated analogs 186. An attempt to prepare 186 directly by base-catalyzed cyclization of 187 afforded a high yield of 2,3-dihydro-6/f-oxepino [3,2-c][l,2]benzothiazin-5(4H)-one 7,7-dioxide (188) (Eq. 39). 

Hydrogen iodide a-Aminocarboxylic acids from acylaminomalonic acid esters with simultaneous cleavage of ethers... 

As pointed out earlier (Chapter 9, Part II), ethers (generically R-O-R, R-O-Ar, and Ar-O-Ar, where R = alkyl and Ar = aryl) can be induced to undergo cleavage reactions with Lewis acids. In particular, hydrogen iodide (HI) found utility as a reagent suited (albeit in low yield and with many side reactions) for... 

Scheme 12.43. A synthesis of tyrosine (Ty r, Y) via the condensation ofp-methoxybenzaldehyde with hippuric acid (A-benzoylglycine) in the presence of acetic anhydride. The reaction is presumed to involve the intermediacy of an azlactone (oxazolone), and the reduction (and ether cleavage) is accomplished with hydrogen iodide (HI) and red phosphorus (see Badshah, A. Khan, N. H. Kidwai, A. R. J. Org. Chem., 1972,37,2916 and references therein.)...

Fig. 2.2. Cleavage of aromatic methyl ether using Sn2 reactions. In the dipolar aprotic solvent DMF, thiolate and chloride ions are particularly good nucleophiles for want of solvation through hydrogen bonding. In pyri-dinium hydrochloride a similar effect occurs because for each chloride only one N5 —H5 group is available for hydrogen bonding. The same increase in nucleophilicityin a dipolar aprotic solvent is used to cleave /i-ketomethyl esters with lithium iodide in DMF (cf. Figure 13.29).

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