Patent GB-709802-A (GB)
Title: Preparation of substituted tetra-hydropyrans
Inventor: HALL REGINALD HAROLD
IPC: C07D309/10
Filed: 1950-08-01
Abstract: The invention comprises substituted tetrahydropyrans of the formula <FORM:0709802/IV (b)/1> wherein R is a primary hydrocarbon radical, R1 and R2 are methyl or ethyl radicals and R3 is a non-aryl hydrocarbon radical.  Such compounds are prepared by reacting a dialdehyde of the formula: <FORM:0709802/IV (b)/2> with an alcohol of the formula R3OH in the presence of an acetalization catalyst.  The dialdehyde may be prepared (as in Specification 706,176) by reaction of the corresponding a -methyl- or ethyl acrolein with the appropriate alcohol in the presence of a basic catalyst, e.g. sodium hydroxide at 0 DEG  to 50 DEG  C.  When the hydrocarbon group R is the same as the hydrocarbon group of the alcohol R3OH, the dialdehyde may be used without isolation, e.g. the substituted tetrahydropyrans may be prepared by treatment of a -methyl- or ethyl acrolein with the alcohol first in the presence of a basic catalyst, and then, after neutralization, with the addition of an acid catalyst.  The preferred starting materials are 2-methoxyand 2-ethoxy methyl-2 : 4-dimethyl-pentane-1 : 5-dials.  R may also be a n-propyl-, n-butyl-, isobutyl-, 2-ethyl hexyl radical, an unsaturated radical, e.g. allyl, cyclo aliphatic, e.g. methylcyclohexyl or an aromatic, e.g. benzyl radical.  The alcohol may be a primary alcohol, e.g. methyl, ethyl, n-propyl, n-butyl, allyl, or 2-ethylhexyl alcohol, cyclohexyl or methyl-cyclohexyl carbinol or benzyl alcohol, or a secondary alcohol, e.g. isopropyl or secondary butyl alcohol.  The acetalization catalyst may be hydrogen chloride, hydrochloric acid, phosphoric acid, sulphuric acid, p-toluene sulphonic acid, or an acid activated earth, e.g. fuller's earth.  In some instances, using an acid activated earth catalyst the reaction proceeds to an intermediate stage (as in example 10 below) and is then completed using a mineral or organic acid catalyst.  The mixing of the reactants is preferably effected at room temperature.  The mixture is then heated, preferably with azeotropic distillation of the water formed.  In examples: 2-methoxymethyl-2 : 4-dimethyl-pentane-1 : 5-dial is reacted with (1) (2) and (6) ethyl alcohol, (3) and (9) methyl alcohol, (7) n-butyl alcohol, (11) isopropyl alcohol, (12) 2-ethylhexanol, to give 3-methoxymethyl - 3 : 5 - dimethyl - tetrahydropyran having 2 : 6-dialkoxy-substituents corresponding to the alcohol used. In examples (4) and (5), 2 - ethoxymethyl - 2 : 4 - dimethyl - pentane-1 : 5-dial is reacted with ethyl and methyl alcohol to give 2 : 6-diethoxy- or 2 : 6-dimethoxy - 3 - ethoxymethyl - 3 : 5 - dimethyltetrahydropyran.  In example (8) a -methacrolein is reacted with methyl alcohol first in alkaline and then in acid p  solution to give 2 : 6-dimethoxy - 3 - methoxymethyl - 3 : 5 - dimethyltetrahydropyran.  In example (10) 2-methoxymethyl - 2 : 4 - dimethyl - pentane-1 : 5-dial is reacted with methyl alcohol and a fuller's earth catalyst to give 6-methoxy-5-methoxymethyl - 3 : 5 - dimethyl - delta2-dihydropyran which is reacted with methyl alcohol in the presence of hydrogen chloride to give 2 : 6 - dimethoxy - 3 - methoxymethyl-3 : 5 - dimethyl - tetrahydropyran.  The products are useful as froth flotation agents.