Compound A, C5H10O, is one of the basic building blocks of nature. All steroids and many other naturally occurring compounds are built from compound A. Spectroscopic analysis of A yields the following information:

IR: 3400 cm–1 1640 cm–1

1H NMR: 1.63 δ (3 H, singlet); 1.70 δ (3 H, singlet); 3.83 δ (1 H, broad singlet); 4.15 δ (2 H, doublet, J = 7 Hz); 5.70 δ (1 H, triplet, J = 7 Hz)

(a) How many double bonds and/or rings do A have?

(b) From the IR spectrum, what is the identity of the oxygen-containing functional group?

(c) What kinds of protons are responsible for the NMR absorptions listed?

(d) Propose a structure for A.