Canadian life insurance giant Sun Life Financial Inc. plans to sell its struggling US annuity unit based in Wellesley and a small portion of its life insurance business to Delaware Life Holdings for $1.35 billion.

The deal, expected to be completed next year, will affect 300 employees in Wellesley, where Sun Life has its US headquarters, and 200 more in Lethbridge, Alberta, and Waterford, Ireland, where Sun Life has operations.