Martinez, Joaquin Royo et al. published their research in Rev. real. acad. cienc. exact. fís. y nat. Madrid in 1952 | CAS: 51488-33-6

3,4-Dimethoxybenzimidamide hydrochloride (cas: 51488-33-6) belongs to ethers. Ether is less polar than esters, alcohols or amines because of the oxygen atom that is unable to participate in hydrogen bonding due to the presence of bulky alkyl groups on both sides of the oxygen atom. At room temperature, ethers are pleasant-smelling colourless liquids. Relative to alcohols, ethers are generally less dense, are less soluble in water, and have lower boiling points. They are relatively unreactive, and as a result they are useful as solvents for fats, oils, waxes, perfumes, resins, dyes, gums, and hydrocarbons. Vapours of certain ethers are used as insecticides, miticides, and fumigants for soil.SDS of cas: 51488-33-6

Amidines in the Diels-Alder synthesis was written by Martinez, Joaquin Royo. And the article was included in Rev. real. acad. cienc. exact. fís. y nat. Madrid in 1952.SDS of cas: 51488-33-6 This article mentions the following:

Several amidines were prepared as follows. CH2(OH)CH2Cl + NaCN → CH2(OH)CH2CN (I), b15 110° (84%); I + finely divided Al (hydroquinone to eliminate polymerization) → CH2:CHCN (II), b. 77.6-79.0° (69.3%); II + EtOH (distilled from Na), saturated with gaseous HCl → CH2:CHC(:NH2Cl)OEt (III), m. 104-4.5° (decomposition), stable in bottles with airtight stoppers; III in absolute EtOH + NH3 in absolute EtOH (3 hr. agitation, followed by standing 2 days) → CH2:CHC(:NH2Cl)NH2. Similarly, vanillin + NaOH + MeI → (MeO)2C6H3CHO → (MeO)2C6H3CH:NOH → (MeO)2C6H3CN, m. 67°, → (MeO)2C6H3C(:NH2Cl)OEt, m. 141° (decomposition) → (MeO)2C6H3C(:NH2Cl)NH2, m. 237°. Piperonal → oxime, m. 110°, → piperonylonitrile, m. 94.5°, → Et piperonylimidate-HCl, m. 122-3°, → piperonylamidine-HCl, m. 238° (decomposition) → piperonylamidine, m. 167°, → picrate, m. 257°. PhCN → m-O2NC6H4CN, m. 117.5°, → m-O2NC6H4C(:NH2Cl)OEt → m-O2NC6H4(:NH2Cl)NH2, m. 250° (decomposition). There was no reaction comparable to a Diels-Alder, between these amidines and styrene, anethole, or maleic anhydride. MeNO2 and CH2(OH)CN, used as solvents, were prepared by the decarboxylation of CH2(NO2)CO2H and by the addition of HCN to CH2O, resp. In the experiment, the researchers used many compounds, for example, 3,4-Dimethoxybenzimidamide hydrochloride (cas: 51488-33-6SDS of cas: 51488-33-6).

3,4-Dimethoxybenzimidamide hydrochloride (cas: 51488-33-6) belongs to ethers. Ether is less polar than esters, alcohols or amines because of the oxygen atom that is unable to participate in hydrogen bonding due to the presence of bulky alkyl groups on both sides of the oxygen atom. At room temperature, ethers are pleasant-smelling colourless liquids. Relative to alcohols, ethers are generally less dense, are less soluble in water, and have lower boiling points. They are relatively unreactive, and as a result they are useful as solvents for fats, oils, waxes, perfumes, resins, dyes, gums, and hydrocarbons. Vapours of certain ethers are used as insecticides, miticides, and fumigants for soil.SDS of cas: 51488-33-6

Referemce:
Ether – Wikipedia,
Ether | (C2H5)2O – PubChem