Add a select a class dropdown in tinymce

To achieve the dropdown styles we have to use the filter hook tiny_mce_before_init. This filter grants developers access to the TinyMCE settings array.

The following example adds custom style options to an existing Style dropdown.

add_filter( 'tiny_mce_before_init', 'my_mce_before_init' );

function my_mce_before_init( $settings ) {

    $style_formats = array(                     
            array(
                    'title' => '1/5 Block',
                    'block' => 'div',
                    'classes' => 'col-md-5',
                    'wrapper' => true
            ),
            array(
                    'title' => 'Pull Left',
                    'block' => 'div',
                    'classes' => 'pull-left',
                    'wrapper' => true
            ),
            array(
                    'title' => 'Button R',
                    'selector' => 'a',
                    'classes' => 'btn btn-default pull-right'
            ),  
            array(
                    'title' => 'Text Center Green Button',
                    'selector' => 'a',
                    'classes' => 'btn btn-green center'
            ),              
            array(
                    'title' => 'Clearfix',
                    'block' => 'div',
                    'classes' => 'clearfix'
            ),
    );

    $settings['style_formats'] = json_encode( $style_formats );

    return $settings;

} 

You can read more about the tinymce buttons and their uses and hooks from here.